Stop packaging test files with the gem

It reduces the size of the gem by half, from 645K to 294K. The default
for new gems (created by bundler) is not to include test files. See
rubygems/bundler#3207

newrelic_rpm.gemspec

@@ -31,7 +31,7 @@ EOS
     "newrelic.yml"
   ]
 
-  file_list = `git ls-files`.split
+  file_list = `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
   build_file_path = 'lib/new_relic/build.rb'
   file_list << build_file_path if File.exist?(build_file_path)
   s.files = file_list