Pull request to correct gem reinstallation for megatest (plus Rails 3)

As a first level (planned) exercise, this runs the mega-test on Rails 3 and Ruby 1.9.3-p392 (both are selected automatically).

Steps to set up:

```
rvm get stable
cd rails_apps_composer
git checkout feature_megatest
       rac_test_RECURLY_API_KEY=Change_this_to_your_key
export rac_test_RECURLY_API_KEY
       rac_test_RECURLY_JS_PRIVATE_KEY=Change_this_to_your_key
export rac_test_RECURLY_JS_PRIVATE_KEY
       rac_test_STRIPE_API_KEY=Change_this_to_your_key
export rac_test_STRIPE_API_KEY
       rac_test_STRIPE_PUBLIC_KEY=Change_this_to_your_key
export rac_test_STRIPE_PUBLIC_KEY
```

The big testing step. (Paste this into the terminal, including the parentheses—all in one go: all of a piece.):

```
(
set -e
bundle update activesupport
gem uninstall rails_apps_composer -x
bundle exec rake reinstall
rm -rf ../generated
bundle exec rake mega:test --trace
)
```