Have dialyzer run on Travis for OTP 17 too

v17 does not have the leading 'R'
```
erl -noshell -eval 'io:format("~p", [erlang:system_info(otp_release)]), erlang:halt(0).'
"17"
```
So the dialyzer wouldn't run in the Travis builds.

And a minor spec fix to appease v17's dialyzer.