Setup benchmarking structure

- Setup dummy app files in `test/dummy`
- Setup dummy test server `bin/serve_dummy
  - Note:  Serializer caching can be completely disabled by passing in
  `CACHE_ON=off bin/serve_dummy start` since Serializer#_cache is only
  set at boot.
- run with
  - ./bin/bench
  - `bin/bench` etc adapted from ruby-bench-suite
  - target files are `test/dummy/bm_*.rb`. Just add another to run it.
  - benchmark cache/no cache
  - remove rake dependency that loads unnecessary files
- remove git gem dependency
  - Running over revisions to be added in subsequent PR

.rubocop.yml

@@ -1,6 +1,7 @@
 inherit_from: .rubocop_todo.yml
 
 AllCops:
+  TargetRubyVersion: 2.2
   Exclude:
     - config/initializers/forbidden_yaml.rb
     - !ruby/regexp /(vendor|bundle|bin|db|tmp)\/.*/