Don't touch the binary when not needed.

This might cause issues, when Bundler uses the git repository, but the
repository is read-only. In such case, although everything is in correct
state, the execution fails with 'Errno::EPERM: Operation not permitted @
chmod_internal' error.
Merge pull request #1057 from voxik/dont-touch-bin-when-not-needed

Don't touch the binary when not needed.