Fixed crash when working directory has spaces

This is a fix for issue #191. I added quotes around a few of the arguments in the calls to `git`. They definitely fixed the error I was experiencing, and should do no harm, though the other invocation of git should be tested as well. I didn't test that case.
One should prefer ruby's `%Q(…)` syntax over escaping quotes for clarity (I thought we had a rubocop rule for that?).

```diff
-        command "git clone \"#{$specs_repo}\" \"#{repo}\" --depth 1"
+        command %Q(git clone "#{$specs_repo}" "#{repo}" --depth 1)
```