Rename hmrc-manuals-frontend to manuals-frontend

Without this deployment fails because the Router data and the assets
prefix won't agree with what's now in Puppet.
Merge pull request #33 from alphagov/rename-hmrc-manuals

Rename hmrc-manuals-frontend to manuals-frontend

Part of https://trello.com/c/c6kTjaZ9/99-manuals-view-a-manual-5