add session management to rails_four_stack

The readme states `Session Management: If a config.session_store is supplied and config.api_only = false, this middleware makes the session available as the session method in ActionController.` which is untrue for rails 4 apps. This commit fixes that.
Fix readme to not use --skip-action-mailer

This option does not exist in the Rails app generator. 
Change the example to skip Active Record instead.
Merge pull request #97 from tamird/patch-1

add session management to rails_four_stack
Merge pull request #104 from Sidane/fix_readme_app_generator_example

Fix readme to not use --skip-action-mailer
Check Rails major >= 4
Use latest rbx and jruby
Test using Ruby 2.1 in Travis
Change Travis configuration to 'rbx' instead of 'rbx-19mode'

Also include related gems according to:
https://github.com/travis-ci/travis-ci/issues/1641