Revert "Add Ruby 2.0 support, fix JRuby"

This reverts commit d01a5fd0380f54adb75cc23f528ec6c84475766f.
Revert "Restore Gemfile"

This reverts commit 9fc38ae87aafbe8642104a7bf69802c2ccfe86ae.
Revert "Add .jruby version suffix"

This reverts commit 5bf92f00d45dd7c68f547424902ecfd586d62c9c.