Added pg and mysql2 to the gemspec.
Added pg and mysql2 to the gemspec.

These gems are development dependencies used for `test:postgres` and
`test:mysql`.

Signed-off-by: Yorick Peterse <82349cb6397bb932b4bf3561b4ea2fad50571f50@gmail.com>

YorickPeterse/zen-cms,YorickPeterse/zen-cms