Define config.eager_load according to standards

Define config.eager_load to false in development and test and set it to
true in production as Rails 4 standards to avoid unwanted entries in the
output.
Add a thin wrapper around runcmd to be DRY in tests

A lot of tests are relying on runcmd just to execute a statement through
Rails' runner the same way. Let's wrap this inside a runner method.
Fix the last_output variable name
Speed up the test suite a bit

We can remove the call to bundle install since we are running rake
through bundler so all the dependencies are already installed.

Moreover, the call to `rake db:create` is necessary only if we are
testing against Rails 3.2 but this is not the case.

Running `time rake test`:

           user     system   total
  Before   178.47s  10.05s   3:47.44
  After    40.91s   4.15s    45.481
Merge pull request #210 from robin850/patch-2

Improve a bit the test suite