Upgrade to new version of Rubygems

[wuz]

This repo will cause a warning when we upgrade to Ruby 2.4.5. Basically the newest Gemspec spec has removed default_executable with no upgrade path:

With help from @rreinhardt9, we narrowed this down and figure we can just remove the default_executable from the .gemspec file, as was done by Rspec (PR here, discussion here).