Add LICENSE to .gemspec

This will include the LICENSE file in the distributed .gem which will help us when distributing the gem as RPM in Fedora [1]. Given the note in README:

```
Released under the MIT license. See `LICENSE` for details.
```
This won't be confusing for people using the final gem file.

Please merge, thank you.

[1] https://fedoraproject.org/wiki/Packaging:LicensingGuidelines