Update virtualenv to 1.11.6

1.7 doesn't appear to play nicely with the version of Python installed
on Ubuntu's trusty.
Only do the symlink if envdir is passed in and you're setting this stuff
up

As it is now we can't use this buildpack in non dyno environments w/o
/app
idempotent symlink

The official interface doc for buildpacks[1] doesn't say
that buildpacks always run in a fresh container or
anything of the like, so we ought to assume that things
we did in previous invocations might stick around.

[1]: https://devcenter.heroku.com/articles/buildpack-api
Merge remote-tracking branch 'upstream/master'