Made Project::SLUG_PATTERN case insensitive

git clone/pull/push fail with `Command seems illegal: git-(receive|upload)-pack` when there is uppercase characters in projects slugs.

This can be fixed by changing `Project::SLUG_PATTERN` to `[a-zA-Z0-9-_]+` as done in this PR, or by adding the `i` flag to this regex : https://github.com/gitonomy/gitonomy/blob/master/src/Gitonomy/Bundle/CoreBundle/Command/GitCommand.php#L96