Remove alpha refs from devise.en.yml

When testing password reset under devise 3 just now, the message 'No account with this email exists.  If you are waiting for an invite, we are rolling them out as soon as possible' appeared.

Have found quite a few references to waiting for invites and to buggy alpha software in devise.en.yml

Have removed these. Couldn't find any reference to them elsewhere in the code, but someone else might wish to look through and confirm this before merging.