Remove coveralls dep

The configuration to publish code coverage metrics to coveralls.io is no
longer present anywhere, so we may as well remove this dep.

scientist.gemspec

@@ -18,6 +18,5 @@ Gem::Specification.new do |gem|
   gem.require_paths = ["lib"]
 
   gem.add_development_dependency "minitest", "~> 5.8"
-  gem.add_development_dependency "coveralls", "~> 0.8"
   gem.add_development_dependency "rake"
 end