Use specific versions for pip requirements from a VCS

[dirkcuys]

Change -e VCS+REPOS_URL[@REV]#egg=PACKAGE to use the @REV tag.

Test with the chosen revisions before committing the requirements file!

See prod.txt and 'pip help install' for more info.

[dirkcuys]

You can use 'pip freeze' to get the specific revision used for the dependencies.