Don't spawn a new shell on `git push` when releasing
Don't spawn a new shell on `git push` when releasing

These `git push` operations are the only ones passing a string argument
to `IO.popen` and those are the exact commands hanging for me when I
try to release. I'm not sure what the reason is (maybe that I use hub,
which does some fancy shell aliasing?), but I think it's a good practice
to avoid a new shell anyways.

mvz/bundler,bundler/bundler,bundler/bundler,mvz/bundler