Actually give up on 1.8.7 compatbility

Gemoji implementation depends on `codepoints` and test depend on some
more Ruby 1.9-only features, so don't pretend that the library will be
1.8 compatible anytime soon. Probably never.

This reverts commit 6a781b647498f06d3a0f1d8ace755a424a5a37fe.
Update gemspec to fix warnings

- Specifying `licenses` is recommended
- More helpful `summary` and `description`