Using double quotes to support Windows command prompt

I'm developing gems in (and for) Windows and the Command Prompt cannot handle single quotes (e.g. `git tag -am 'v1.0.0' v1.0.0` fails with `fatal: Failed to resolve 'v1.0.0' as a valid ref.`). I could use Git Bash that comes with msysGit, but it's also an easy fix to support Command Prompt with this change. Double quotes are supported on all platforms.

Here's a relevant discussion from 2009: https://groups.google.com/forum/#!topic/msysgit/qzKNBrjYUUM/discussion