Check ruby-prof

In Ruby2.0.0, ```bundle``` does not install ruby-prof, so I check required or not in ```activesupport/lib/active_support/testing/performance/ruby.rb```, and I add 2spaces for indent.

See /Gemfile
```
gem 'ruby-prof', '~> 0.11.2' if RUBY_VERSION < '2.0'
```