Add MIT license to Gemspec

We had the license file, but we didn't have it listed in the gemspec.

Issue: https://github.com/causes/mock_redis/issues/64

Change-Id: Ibadb34b3c7825774dfbc5bffd3c6fdd611b05429
Reviewed-on: https://gerrit.causes.com/26336
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Aaron Neyer <aaron.neyer@causes.com>
Tested-by: Aaron Neyer <aaron.neyer@causes.com>