MX Player 1.9.1 is not available to everyone. It's released to those people who has opted to be an alpha tester in Google Play. So, we do not recommend it to install if you don't want to be a tester. You can get the test builds from our official page "https://sites.google.com/site/mxvpen/translation/test-build" rather than using a third party repo.
Regarding Pro, It's just a test build for alpha testers. We will roll out the same to the pro version if there is no major issue reported from our alpha testers.