Test against Ruby 2.2.4 on Travis.
Test against Ruby 2.2.4 on Travis.

ngpestelos/rack,gioele/rack,ngpestelos/rack,gfvcastro/rack,wjordan/rack,gfvcastro/rack,wjordan/rack,imtayadeway/rack,gfvcastro/rack,rwz/rack,rwz/rack,gioele/rack,rwz/rack,ngpestelos/rack,Eric-Guo/rack,imtayadeway/rack,gioele/rack,imtayadeway/rack,Eric-Guo/rack,wjordan/rack,Eric-Guo/rack