Get ndkVersion from main projects' gradle config (nodejs-mobile#14)

Author: siepra

android/build.gradle

@@ -51,6 +51,8 @@ android {
     compileSdkVersion ((rootProject?.ext?.properties?.compileSdkVersion) ?: 23)
     buildToolsVersion ((rootProject?.ext?.properties?.buildToolsVersion) ?: "23.0.1")
 
+    ndkVersion project(":app").android.ndkVersion
+
     defaultConfig {
         minSdkVersion _nodeMinSdkVersion
         targetSdkVersion _nodeTargetSdkVersion