[READY] Use codecov bash uploader on Travis

Since we added C++ coverage with PR #636, our coverage reports are stuck with the following message:
```
Waiting for CI to finish and reports to complete processing. Please review commit coverage with caution, it may change.
```
This is caused by one of our Travis build (Linux, Python 3.3) that fails to upload coverage to codecov with this Python encoding error:
```
Error: 'latin-1' codec can't encode character '\u2013' in position 14079665: ordinal not in range(256)
```
I've contacted codecov support about this error and they suggested to use the bash uploader instead of the Python one.

Since the bash uploader cannot be used on AppVeyor, we still need `codecov` but not as a test dependency so we remove it from `test_requirements.txt` and install it directly in the AppVeyor script.

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[READY] Update ycmd

# PR Prelude

Thank you for working on YCM! :)

**Please complete these steps and check these boxes (by putting an `x` inside
the brackets) _before_ filing your PR:**

- [X] I have read and understood YCM's [CONTRIBUTING][cont] document.
- [X] I have read and understood YCM's [CODE_OF_CONDUCT][code] document.
- [X] I have included tests for the changes in my PR. If not, I have included a
  rationale for why I haven't.

> ycmd tests itself

- [X] **I understand my PR may be closed if it becomes obvious I didn't
  actually perform all of these steps.**

# Why this change is necessary and useful

A number of enhancements have been made in the server. They seem stable in my testing. Let's push them live now.

Includes all of the following PRs:

* Auto merge of ycm-core/ycmd#631 - vheon:refactor-request-wrap, r=micbou: [READY] Add 'first_filetype' to RequestWrap
* Auto merge of ycm-core/ycmd#634 - vheon:fix-previous-identifier, r=micbou: [READY] Avoid to wrap around when searching for previous identifier
* Auto merge of ycm-core/ycmd#635 - micbou:resource-warning, r=Valloric: [READY] Fix warnings when resetting process handle
* Auto merge of ycm-core/ycmd#637 - vheon:refactor-tags-file, r=micbou: [READY] Extract filtering of tags as a generator
* Auto merge of ycm-core/ycmd#639 - mixedCase:tern_0.20, r=micbou: Bump Tern to version 0.20
* Auto merge of ycm-core/ycmd#640 - vheon:test-identifier-completer, r=micbou: [READY] Additional test for the identifier completer
* Auto merge of ycm-core/ycmd#636 - puremourning:coverage-c++, r=micbou: [READY] Coverage for c++ code
* Auto merge of ycm-core/ycmd#641 - micbou:remove-tests-folder, r=vheon: [READY] Remove unused tests folderS
* Auto merge of ycm-core/ycmd#643 - micbou:javascript-identifiers, r=Valloric: [READY] Add regex for JavaScript and TypeScript identifiers
* Auto merge of ycm-core/ycmd#644 - micbou:flake8, r=Valloric: [READY] Fix flake8 errors
* Auto merge of ycm-core/ycmd#645 - micbou:global-config-exception, r=Valloric: [READY] Catch exceptions from global extra conf
* Auto merge of ycm-core/ycmd#649 - micbou:codecov, r=puremourning: [READY] Use codecov bash uploader on Travis
* Auto merge of ycm-core/ycmd#652 - micbou:remove-executable-mode, r=Valloric: [READY] Remove executable mode
* Auto merge of ycm-core/ycmd#650 - vheon:remove-unused-assert, r=micbou: Remove unused CustomAssert
* Auto merge of ycm-core/ycmd#651 - micbou:global-extra-conf-reuse, r=Valloric: [READY] Do not reload extra conf file
* Auto merge of ycm-core/ycmd#657 - micbou:gocode-submodule, r=Valloric: [READY] Update Gocode submodule (Fixes #2449)
* Auto merge of ycm-core/ycmd#665 - micbou:flake8, r=micbou: [READY] Do not disable Flake8 on Python 2.6 Travis but restrict its version
* Auto merge of ycm-core/ycmd#667 - haifengkao:SupportObjcImportModule, r=Valloric: Support objc import module
* Auto merge of ycm-core/ycmd#677 - puremourning:dev-flags, r=micbou: [READY] Enable dev flags when --enable-debug is supplied

[cont]: https://github.com/Valloric/YouCompleteMe/blob/master/CONTRIBUTING.md
[code]: https://github.com/Valloric/YouCompleteMe/blob/master/CODE_OF_CONDUCT.md

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