Update build.gradle (#9)

Author: youngjuning

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("buildToolsVersion", "23.0.1")
 
     defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 22
+        minSdkVersion safeExtGet('minSdkVersion', 16)
+        targetSdkVersion safeExtGet('targetSdkVersion', 22)
         versionCode 1
         versionName "1.0"
     }
@@ -33,4 +37,4 @@ repositories {
 dependencies {
     compile 'com.facebook.react:react-native:+'
 }
-  
+