Fix the inappropriate Play Services update request

The issue is with '+' dependency references. The Fit Preview Play Services (v5.2) gets picked by Gradle as it's available on the user's local repo, but it's not available on devices (except for devices with the L Fit Preview). Devices out there only have 5.0.89 available.

Swapping out all '+' dependencies, the app now correctly works and builds no matter what the environment is. This is also recommended by the new Google guidelines.

This fixes issue #37 and #38.