req: Support --no-use-wheel flag in requirements.txt

Default from pip 1.6 will be to always use wheels, so the --use-wheels
in requirements.txt is now deprecated. However, the --no-use-wheels
is introduced to be able to disable wheel usage via requirements.txt.
Document the --no-use-wheel in requirements.txt
Merge pull request #1859 from nailor/support-no-use-wheel

req: Support --no-use-wheel flag in requirements.txt