Bump rspec dependency to cover all future 3.x releases

Background:

* We hook deep into RSpec internals to get very fine grained test
  selection
* These APIs are NOT public.
* Hence mutant was always very restrictive in its versioning, to not
  whitelist a version where these internals are eventually changed.
* No such change happened in years.

Conclusion:

* The past approach of whitelisting rspec version after rspec version is
  not the correct choice for reducing the mutant maintainership
  workload.
* We'll eat the potential regression over not automatically covering new
  rspec releases in the 3.x series (yeah, I know potentially last famous words).

mutant-rspec.gemspec

@@ -16,7 +16,7 @@ Gem::Specification.new do |gem|
   gem.extra_rdoc_files = %w[LICENSE]
 
   gem.add_runtime_dependency('mutant', "~> #{gem.version}")
-  gem.add_runtime_dependency('rspec-core', '>= 3.4.0', '< 3.8.0')
+  gem.add_runtime_dependency('rspec-core', '>= 3.4.0', '< 4.0.0')
 
   gem.add_development_dependency('bundler', '~> 1.3', '>= 1.3.5')
 end