Use the correct rails path in `rake engine_cart:create_test_rails_app`

[jcoyne]

The code uses with_clean_env, but it probably should memoize the current rails path before doing that.

https://github.com/cbeer/engine_cart/blob/52f2dbfc9428f3b9ee9a43ff81578e2515fca3f7/lib/engine_cart/tasks/engine_cart.rake#L26-L28

Bundler could not find compatible versions for gem "rails":
  In Gemfile:
    spotlight (>= 0) ruby depends on
      rails (~> 4.0.1) ruby

    rails (4.1.0.beta1)

[jcoyne]

This seems to be a very difficult problem to solve. The bin/rails does not change between versions of rails. RAILS_GEM_VERSION, no longer seems to be part of rails and the old hack to do rails _3.0.2_ new foo doesn't work any longer either.

[cbeer]

Fixed in 3613187de914d354e13f6fcfb5fd532b43165664