Reject Ace modes that are lacking a mode
Set missing `ace_mode` values to `text`

This makes it much easier to deal with when iterating over all the
languages, since `text` is a valid, non-highlighting lexer.
This `reject` is no longer necessary
Update test to account for using `text`
Merge branch 'master' into return-nil-on-no-ace-mode
Set text's ace_mode to text
Add `warn` message indicating deprecation
Merge branch 'master' into return-nil-on-no-ace-mode
Set Public Key ace_mode to text
Merge pull request #1831 from github/return-nil-on-no-ace-mode

Set "missing" ace modes to `text`