Use Google's default repository for extension-api gradle dependencies.

Jcenter is the default for Android Studio projects, is a superset of Maven Central, and is what is used in the default lime template.

Fixes Android builds failing to resolve gradle and gradle plugin versions that are missing from Maven Central.

dependencies/extension-api/build.gradle

@@ -1,7 +1,7 @@
 buildscript {
 	repositories {
-		maven {
-			url "http://repo1.maven.org/maven2/"
+		jcenter {
+			url "http://jcenter.bintray.com/"
 		}
 	}