Use #drop instead of a bit less readable [1..-1]
Do not test against old 2.1.x Rubies on Travis
Remove unneeded files
Update ruby header files for 2.2.0dev
Remove duplicate ruby header files

 * 2.1.2 uses the same header files as 2.1.1
 * same for 2.1.4 and 2.1.5
Force to upgrade this gem to get it working with new Ruby versions

To avoid introducing a bug like https://github.com/yuki24/did_you_mean/issues/14,
did_you_mean shouldn't be able to compile without the latest ruby header
files if a new Ruby version is released.