Support added for Rails 4.2.

`Rails 4.2.0.beta2` has updated `rack` to `1.6.0.beta`, so `rack` need change the version to `>= 1.5.2`.

	Bundler could not find compatible versions for gem "rack":
	  In Gemfile:
	    rack-tracker (~> 0.2.3) ruby depends on
	      rack (~> 1.5.2) ruby
	
	    rails (= 4.2.0.beta2) ruby depends on
	      actionpack (= 4.2.0.beta2) ruby depends on
	        rack (1.6.0.beta)