Replace explicit files enumeration with glob in gem spec

Try to prevent errors like codecov#119.

codecov.gemspec

@@ -8,7 +8,7 @@ Gem::Specification.new do |s|
   s.summary               = 'Hosted code coverage'
   s.description           = 'Hosted code coverage Ruby reporter.'
   s.email                 = ['[email protected]']
-  s.files                 = ['lib/codecov.rb', 'lib/codecov/version.rb']
+  s.files                 = Dir['lib/**/*.rb']
   s.homepage              = 'https://github.com/codecov/codecov-ruby'
   s.license               = 'MIT'
   s.platform              = Gem::Platform::RUBY