Don't exclude .git on the slug

_From @crigor on June 25, 2015 23:0_

Is there a reason the builder still excludes .git? https://github.com/deis/deis/blob/3cc3baf0fe7084bfe11b1dad47dc377879a14a2d/builder/rootfs/usr/local/src/slugbuilder/builder/build.sh#L155

When using a github url as a dependency on an elixir app, the .git dir is needed. The app worked fine on heroku but failed to start on deis.

_Copied from original issue: deis/deis#3924_