Specify API key in gradle.properties

[gb96]

The testfairy API key should not be visible inside the build.gradle, particularly for an open-source project. Instead the key should be specified in a gradle.properties file that is not included in version-control.

I had a few problems getting this to work correctly, the main issue was putting gradle.properties in the correct directory (can be either in the same directory as build.gradle or in USER_HOME/.gradle). I also had a problem with the double-quotes around my API key. When you move the key into a gradle.properties you must remove the double quotes around it otherwise the key is rejected.