[1927] Get Travis CI working

For #1927

## Changes

### Misc
- reordered Gemfile -- shouldn't change functionality
- fix test env builder to support Rails versions properly

### Issues
- #1966
  - fix missing `decorator_method` for Draper
- #1967
  - remove 1.9 hash syntax
  - fix I18n dynamic re-translation feature

### Travis
- use latest Rails maintainence releases
- use latest Bundler version
- remove REE & 1.9.2
- add 2.0.0

## Travis build history from before commit squash
* 04dc755 [before Bundler update](https://travis-ci.org/gregbell/active_admin/builds/5298607)
* f49d884 [after Bundler update](https://travis-ci.org/gregbell/active_admin/builds/5299819)
* 6f77525 [after fixing cukes for #1966](https://travis-ci.org/gregbell/active_admin/builds/5330322)
* 227c07c [after removing 1.9 hash syntax that snuck in from #1967](https://travis-ci.org/gregbell/active_admin/builds/5334575)
* e86e648 [after fixing REE KeyError issue](https://travis-ci.org/gregbell/active_admin/builds/5393192)
* b3b2c4e [after resolving dynamic i18n re-translation issue caused by #1967](https://travis-ci.org/gregbell/active_admin/builds/5457187)
* 5f8e024 [after fixing Rails 3.2 mass assignment error](https://travis-ci.org/gregbell/active_admin/builds/5477321) Note: Bundler updated to 1.3.3 and broke REE
* 476f8e4 [after removing REE and 1.9.2, and adding 2.0.0 to Travis](https://travis-ci.org/gregbell/active_admin/builds/5478222)