Revert "bump v.1.28"

This reverts commit b19c106521e35fc267294314f297685d3f15aa37.
Revert "update Gemfile"

This reverts commit 31b32c5e55828e34266bdff84b15335d80c31229.
Revert "bump v.1.28.beta.1"

This reverts commit c3b9617e567371c84ddf0d209924adc1676f2297.
Revert "jruby 1.7.15"

This reverts commit d4030baa171e65914c5a99c67a1d687991034473.