[bug 1107809] Fix Gengo language guesser

On Thursday, Gengo did a massive API update and in the process the
unofficial language guesser API endpoint we were using disappeared.

This updates the url to the correct url, tweaks the code to handle the
new response shape and switches us to use HTTPS.
Merge pull request #402 from willkg/1107809-fix-guesser

[bug 1107809] Fix Gengo language guesser