make git version available to the build process

If the user sets config var GO_GIT_DESCRIBE_SYMBOL, we
add a linker flag to assign the output of 'git describe'
to the specified symbol.

closes #50
use commit id for version symbol if no tag exists

This makes GO_GIT_DESCRIBE_SYMBOL useful even if no tags
have been pushed to Heroku.
omit needless words
Update virtualenv to 1.11.6

1.7 doesn't appear to play nicely with the version of Python installed
on Ubuntu's trusty.