Move encoding classes setup later, so fixnum constants can be set.
Improve assertions for bad constant name/value.
Migrate toward new spec:ruby rake targets for RubySpec.
Upgraded RSpec to latest version
Define missing Converter constants and untag passing specs.
Merge pull request #883 from prathamesh-sonpatki/upgrade-gems

Upgrade RSpec to latest version
Implement Encoding::Converter.asciicompat_encoding.

I hand-set up the mapping for the moment to get things running,
but I assume there's a table somewhere in jcodings we can use to
do this programmatically.