Explicitly enable public action request formats

Instead of throwing 500 errors for requests for formats that are not
supported, return a 406. Actions that need to respond to formats other
than html must now explicitly enable them using the
enable_request_format helper.
Merge pull request #46 from alphagov/reject-not-acceptable-requests

Explicitly enable public action request formats
Upgrade to Ruby 2.1.2
Install launch to support `save_and_open_page` in cucumber.
Add Subcategory#combined_title.

Provides context for short topic titles like
"Wells" in "Oil and Gas: Wells".
Expose Subcategory#slug.
Bump the govuk_frontend_toolkit to 1.7.0
Bump the gds-api-adapters to 15.0.0
Alphabetise Sass imports.
Allow signup to email alerts for topics.
Merge pull request #47 from alphagov/allow-signup-to-email-alerts-for-topics

Allow signup to email alerts for topics
Provide a shortcut email signup URL.
Merge pull request #48 from alphagov/shorter-email-signup-url

Provide a shortcut email signup URL.
Disable CSRF checking for email signups.
Merge pull request #49 from alphagov/disable-csrf-checking-for-email-signups

Disable CSRF checking for email signups.