Use private .ruby-* files instead of .rvmrc
No need to depend on json for modern versions of JRuby

Since JRuby 1.6 can’t even be installed easily anymore there’s no loss in not supporting it out of the box.
Use https for RubyGems
Fix metrics-core-jars version to 2.1.2

The gem stopped to work after this version.
Merge branch 'pr_6'