set secret_key_base if not provided

assets:precompile rake task does set MAS_RAILS_SECRET_TOKEN

This is needed by devise which was previously not loaded as behind
feature toggles
Merge pull request #789 from moneyadviceservice/build-secret

precompile assets requires MAS_RAILS_SECRET_TOKEN
Remove validates_timeliness

This library has been abandoned, has been producing deprecation warnings
and is only used to validate a single field so remove it and perform the
validation ourselves