Add byebug

* An interactive debugger is an essential tool for debugging behavior
* byebug is the most stable and usable debugger on Ruby 2
Upgrade Capybara and Capybara Webkit

The previous stable release of capybara-webkit didn't support Capybara 2.3. We
just released capybara-webkit 1.2, with support for the latest version.
Remove Pow in bin/setup

Our primary issue is that we do not have development/staging/production parity
when using Pow locally. Pow doesn't use event machine or a forking model, so:

* Projects using Thin will work locally but not work in production if they use
  threads or gems with eventmachine incorrectly
* Projects using Unicorn will work locally but not in production if process
  spawning is not managed correctly

Using Pow in development and something else in production would be like using
single threaded code in development but multiple threads in production; we'll
end up with race conditions eventually.

Pow is more different from production than Foreman is (at least Foreman uses the
same web server), and it's likely to be different from other members of the
team.

We'd prefer to have only one way of running the app within a project and we'd
prefer to have that way be as close to production as possible.

Keeping it the same reduces the likelihood that somebody breaks the app for
everybody else because of a "works on my machine" scenario. It also reduces the
number of onboarding questions, because everybody sets up and runs the app the
same way: `bin/setup` and `foreman start`.
Remove Campfire in favor of Slack

We don't need a placeholder for Slack because we'll typically add it using the
CLI:

    travis encrypt "thoughtbot:123abc" --add notifications.slack

http://blog.travis-ci.com/2014-03-13-slack-notifications/
Update Rails version to 4.1.4

http://weblog.rubyonrails.org/2014/7/2/Rails_3_2_19_4_0_7_and_4_1_3_have_been_released/
http://weblog.rubyonrails.org/2014/7/2/Rails_4_0_8_and_4_1_4_have_been_released/

Fix the location of the assets version, which has changed. Use a default value
for `ENV['ASSETS_VERSION']` to make testing and other environments easier to
work with but prioritize the ENV variable so we can bust the cache in production
through changing the ENV variable without deploying new code.
Use latest thoughtbot/guides, houndci.com styles

We normally don't edit existing code to match the style guide, but I think it's
better to edit the generated code once here rather than every time we generate
the app.

The most widespread change is double quotes in Ruby:

https://github.com/thoughtbot/guides/commit/2e9733d1cd73243164548673427e79ed3482ee7e
v1.15.0
Upgrade to rspec-rails 3.x.x

    config.infer_spec_type_from_file_location!

Gives us spec types for free, like `type: :controller` or `type: :feature`.

    config.treat_symbols_as_metadata_keys_with_true_values = true

Defaults to true in RSpec 3.
Remove Brewfile, document laptop setup

There was a Brewfile template in the Suspenders source that was not being used
when generating an app. Rather than maintaining it, it might be better to
document for app developers that they should set up their machine using
something like thoughtbot/laptop.
Remove Foreman from Gemfile

Provide documentation in project's README and in `bin/setup`.

https://github.com/ddollar/foreman/pull/437#issuecomment-41110407
Raise on missing translations in development/test
Run suspenders only once
Add uncomment_in_file helper
Better variable name