Upgrade gem dependencies
Fix no freeze shared spec allow frozen input

* Bignums under 2.1 are frozen by default
Rename shared specs to avoid duplicate inclusion

* Rspec will load any file suffixed with `_spec.rb` we load shared specs
  via devtools so duplicate inclusion can be avoided via removing the
  `_spec.rb` suffix.
Fix specs not to freeze nil
Fix specs to not access uninitialized ivars
Fix cache lookup to not race on cache write
Disable mutant on rake ci
Remove jruby-head from travis matrix

* Travis seems to set an JVM property jruby.cext.enabled that is not
  suppported on jurby-head anymore.
Fix rubocop configuration
Upgrade ruby 2 version for travis to latest stable
Merge pull request #18 from mbj/fix-cache-race

Fix cache race