Fix sum to use inject(:+)
Fix rubocop and mutant to not raise an error under ruby 2.1
Upgrade gem dependencies
Add jruby-head to the list of rubies that are allowed to fail

* This should hopefully be a temporary fix
Remove ruby-head from list of rubies allowed to fail

* Some long-standing issues have been resolved allowing the build to
  finally pass with ruby-head again.
Move Flay spec to integration specs directory where it's belong
Fix failing spec for Devtools::Task::Flay.run_flay
Rename Devtools::Task to Devtools::Tasks