psycopg2 wheel gets installed in spite of the --no-binary psycopg2 option.

Environment Versions

1. OS: Debian
2. Python version: 3.6.4
3. pip version: 9.01
4. pip-tools version: 1.11.0

Steps to replicate

1. Create deps.in with line psycopg2 --no-binary psycopg2
2. Run pip-compile deps.in, see that the flag is preserved in the txt file
3. Run pip-sync deps.txt

Expected result

A psycopg2 sdist is installed. pip install -r deps.txt does that.

Actual result

A psycopg2 wheel is installed.