Only re-define default rake task when RSpec available.

On e.g. production systems it isn't:

       rake aborted!
       NameError: uninitialized constant RSpec
       /home/hammock/rails/tmp/build-140802803216559/Rakefile:12:in `block in <top (required)>'
       /home/hammock/rails/tmp/build-140802803216559/Rakefile:10:in `<top (required)>'
Update README
Divide spec_helper into rails_helper and spec_helper.
Correction
Add note about smusher
Add height: 100% to HTML tag
Mina-rollbar
Update Gemfile.lock and fix specs accordingly
Merge branch 'master' of github.com:jmuheim/base