Remove "else" condition that is never hit

Literally, the code said "campaign = render ..." which is not just
wrong, but also invalidates the successive line that raises an
error if a campaign is not set
Don't make merge_vars a required param

If merge_vars is not set, then it should be set to an empty hash.
Setting it as nil breaks the method that calls it, which is always
expecting a Hash.
Remove unused send_params method

Not called anywhere
Remove unused Language.select_args method
Move language_select_args from model to Admin code

@@language_select_args is only used in the view to list the available ISO 639 language, therefore it does not belong in the Language model
Remove unused Campaign#enabled? method
Move views-only methods from models to helpers

Methods that affect Language and Campaign only in the way in
which they are displayed in the ActiveAdmin views belong to
the helpers, not to the model themselves.
Replace "abstract" specs with descriptive specs

What does the whole Mercury app do? Three things:

* POST /messages and a message is sent to Mandrill
* GET /templates/:id/preview and you preview a template
* GET /layouts/:id/preview and you preview a layout

The previous specs did not really ensure these behaviors and spent
a lot of time in dev-only details.

The new specs go through the entire flow and achieve 100% coverage
for the user-facing app (leaving the ActiveAdmin part un-specified).
Remove commented and unused gems from Gemfile
Bump Rails to 4.1.1

Note that activeadmin must be bumped too since the previous version
did not accept Rails 4.1 as a valid dependency
Remove blank lines and empty files
Remove duplicate code (render html / text)
Merge pull request #3 from Fullscreen/better-code

Better code