Upgrade mocha version to 5.2.0 in all packages. (#1375)
Upgrade mocha version to 5.2.0 in all packages. (#1375)

We are trying to consolidate flags passed to mocha test
into mocha.opts (a mocha config file). In order for mocha
to support comments in this file, mocha needs to be upgraded
to its latest version (5.2.0 as of 11/09/2018).
FirebaseExtended/rxfire,FirebaseExtended/rxfire,FirebaseExtended/rxfire