Fix a warning for frozen string literal

Just fixing following warning:
```
❯ RUBYOPT=-w bundle exec rake test
/ydah/racc/Rakefile:80: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)
```

Author: ydah

Rakefile

@@ -77,7 +77,8 @@ else
   # MRI
   require "rake/extensiontask"
   extask = Rake::ExtensionTask.new "cparse" do |ext|
-    lib_dir = (ext.lib_dir << "/#{RUBY_VERSION}/#{ext.platform}/racc")
+    ext.lib_dir = ext.lib_dir + "/#{RUBY_VERSION}/#{ext.platform}/racc"
+    lib_dir = ext.lib_dir
     ext.ext_dir = 'ext/racc/cparse'
   end
 end