Changed log

• Add php-7.2 and php-7.3 tests on Travis CI build.
• Removing --dev option because this option is deprecated on latest composer version.
  The deprecated warning message is as follows:

You are using the deprecated option "dev". Dev packages are installed by default now.

• Using the assertNull to assert that the expected value is same as null.
• According to the .travis.yml setting, the minimal PHP version test is 5.4.
  Modify the php version requirement to be >=5.4 inside require block on composer.json.