Add constrain for /admin route

Sets a flag in development.rb to enable admin routes. This should be set
to true on a backend box as part of the deploy process.

https://www.pivotaltracker.com/story/show/67458214
Merge pull request #99 from alphagov/constrain-routes-for-admin

Add constrain for /admin route
Fix for test env breaking with admin constraints

After 6f162d2301981b4c9268fb48ec5e980c68bc28bc, tests won't work without
due to enable_admin_routes not being set. This commit fixes that.
Merge pull request #101 from alphagov/fix-admin-routes-in-test-env

Fix for test env breaking with admin constraints
Merge commit '5877bc168d9795a6e62fa235fe64cb7e7cf3249c' into HEAD