Merge pull request #22 from alphagov/meta-viewport

Allow apps to define removal of the meta viewport
Bump to version 3.7.0

Add ability to turn off viewport meta tag via the `X-Slimmer-Remove-Meta-Viewport` header.
Campaign replacement is a header setting

Allow apps to explicitly state whether they want to replace campaign
content.
Use the constant that we've set elsewhere

Rather than declaring an explicit string and make this harder for others
to refactor later, use the constant set in the headers file.
Add a notification inserter processor

This enables apps to explicitly add a processor call to insert/update
their campaigns depending on the campaign provided by static.
Hook up the CampaignNotificationInserter

When a successful render occurs, show the campaign if the correct header
is set.
Add a campaign_notification header

This is the specific header that apps will set to turn on/off whether
they want this functionality.
Revert "Add a campaign_notification header"

This reverts commit 201491c249e1d8de6ca36d5dd6ce415392191fcd.
Use the processor style used elsewhere

Rather than creating a custom post-processor, add this process to the
array to be worked on as part of the Slimmer workflow.
Remove stubbing from setup

Handling in a similar way to other processors now using per-class
stubbing.
Don't use nokogiri when a raw string will do
Use the provided test helper method for template

Instead of stubbing out templates explicitly, use the test helper
template method to use a fixture for all processing cases.
Remove the response block that's not used
Add a test case helper for campaigns
Add a separate else case for campaign templates
Update else case so it's actual Ruby
Merge remote branch 'origin/campaign-notifications'
Bump version to 3.8.0

This is to pickup the new CampaignNotificationInserter processor that
was added.
Added ability to reference organisations to header

This is so that from within Whitehall we can specifically reference an
organisation set and have those appear as GA custom variables for
Analytics.
Removed unneeded downcase