Restrict RDiscount dependency to the MRI platform

It cannot be installed under JRuby because of C extensions.
Add support for the latest master of JRuby

Fall back to monkeypatching String because refinements are not yet
implemented on JRuby.
Update project history
Bump version number
Tweak Travis CI configuration to specify Ruby versions consistently

Specifying patch versions feels too specific -- minor versions is
better.
Merge branch 'master' into new-api