Revert "Add requests requirement capped <1.2.1."

Fixes bug 1182271.

This reverts commit c39687edb9138c6eac675b8781a5d4bf5a51c968. The
original workaround is no longer needed, as pip is now configured to
install with the --upgrade flag and will therefore pull a new enough
python-jinja2 into the tox virtualenv even though site packages is
enabled in nova and an older distribution package of the same
library is still present on CentOS 6.4.

Change-Id: I33335314c7c1b26d630137edfa0cdc2d1947432e
Merge "Revert "Add requests requirement capped <1.2.1.""