Update test environment related files boot.rb, Rakefile, Readme...

This pull request fixes some of the problem I encountered trying to run locally the test suite to contribute something. @alup tried to help me here https://github.com/RubyMoney/money-rails/issues/161 and I found the bug to be located here: https://github.com/RubyMoney/money-rails/blob/master/spec/dummy/config/boot.rb#L5

What was happening was that the Rakefile passed BUNDLE_GEMFILE env variable as being overwritten all the time, even if it was already set.

Besides fixing this, I simplified the Rakefile a little bit, remove the Activerecord task that I didn't get to understand why it had to be there or how it'd execute correctly. I added some comments in the Readme to help future contributors. I tried cloning the repo and following the instructions from scratch with rails 3 & 4 (no mongod in my machine) and it all works.

I'm working on verifying a possible bug related to currencies where the decimal separator isn't a dot, but that's another pull request (if I prove the issue to be real).

Let me know if there's something you think should be made different, I tried my humble best but I'm totally open to feedback and improvement.