Update build.gradle

Solves issue #29

Author: António Goulão

android/build.gradle

@@ -11,13 +11,17 @@ buildscript {
 
 apply plugin: 'com.android.library'
 
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 android {
-    compileSdkVersion 23
-    buildToolsVersion "23.0.1"
+   compileSdkVersion safeExtGet("compileSdkVersion", 23)
+    buildToolsVersion safeExtGet("compileSdkVersion", "23.0.1")
 
     defaultConfig {
         minSdkVersion 16
-        targetSdkVersion 22
+        targetSdkVersion safeExtGet("compileSdkVersion", 22)
         versionCode 1
         versionName "1.0"
     }