Merge pull request #129 from yeomann/patch-1

improvement way to get root SDK versions

android/build.gradle

 apply plugin: 'com.android.library'
 
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 repositories {
     mavenCentral()
 }
 }
 
 android {
-    compileSdkVersion 23
-    buildToolsVersion "23.0.1"
+    compileSdkVersion safeExtGet('compileSdkVersion', 26)
+    buildToolsVersion safeExtGet('buildToolsVersion', '26.0.3')
     defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 23
+        minSdkVersion safeExtGet('minSdkVersion', 16)
+        targetSdkVersion safeExtGet('targetSdkVersion', 26)
         versionCode 1
         versionName "1.0"
     }
 }
 
 dependencies {
-    compile 'com.facebook.react:react-native:+'
+    compile "com.facebook.react:react-native:${safeExtGet('reactNativeVersion', '+')}"
     //compile 'com.squareup.okhttp3:okhttp:+'
     //{RNFetchBlob_PRE_0.28_DEPDENDENCY}
 }