Update build.gradle

Manually merge this PR mauritsd#7 to fix android version problem

Author: YoungForest

android/build.gradle

@@ -1,3 +1,4 @@
+apply plugin: 'com.android.library'
 
 buildscript {
     repositories {
@@ -9,15 +10,22 @@ buildscript {
     }
 }
 
-apply plugin: 'com.android.library'
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
+def DEFAULT_COMPILE_SDK_VERSION = 23
+def DEFAULT_BUILD_TOOLS_VERSION = "23.0.1"
+def DEFAULT_MIN_SDK_VERSION = 16
+def DEFAULT_TARGET_SDK_VERSION = 22
 
 android {
-    compileSdkVersion 23
-    buildToolsVersion "23.0.1"
+    compileSdkVersion safeExtGet('compileSdkVersion', DEFAULT_COMPILE_SDK_VERSION)
+    buildToolsVersion safeExtGet('buildToolsVersion', DEFAULT_BUILD_TOOLS_VERSION)
 
     defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 22
+        minSdkVersion safeExtGet('minSdkVersion', DEFAULT_MIN_SDK_VERSION)
+        targetSdkVersion safeExtGet('targetSdkVersion', DEFAULT_TARGET_SDK_VERSION)
         versionCode 1
         versionName "1.0"
     }
@@ -31,5 +39,5 @@ repositories {
 }
 
 dependencies {
-    compile 'com.facebook.react:react-native:+'
+    implementation 'com.facebook.react:react-native:+'
 }