Use quotes for commit message, not apostrophes

Using apostrophes instead of quotes in some environment seems to cause git to attempt to evaluate the commit message and throw the following errors:

```
[2018-08-02 17:05:04] staging.ERROR: Spock command exited unsuccessfully:
Command: git commit -m 'Page saved by user'
Output: No output
Error: 
error: pathspec 'saved' did not match any file(s) known to git.
error: pathspec 'by' did not match any file(s) known to git.
error: pathspec 'user'' did not match any file(s) known to git.
```

The change in this commit should resolve the issue.

addons/Spock/Git.php

@@ -36,7 +36,7 @@ public function commands()
             $commands[] = "git add {$path}";
         }
 
-        $commands[] = vsprintf("git commit -m '%s%s'", [
+        $commands[] = vsprintf('git commit -m "%s%s"', [
             $this->label(),
             $this->user ? ' by ' . $this->user->username() : ''
         ]);