change versionLabel to 2.0 (fit new major.minor scheme)

Pebble-authenticator currently fails to compile due to the newer versions of the Pebble SDK enforcing a minor.minor versioning scheme.

    Exception: appinfo.json versionLabel format for app revision must be "Major" or "Major.Minor"

This patch changes the versionLabel to 2.0 in order to allow it to compile.