update android compileSdkVersion and buildToolsVersion to 26

android/build.gradle

@@ -11,9 +11,13 @@ buildscript {
 
 apply plugin: 'com.android.library'
 
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 android {
-    compileSdkVersion 25
-    buildToolsVersion "25.0.3"
+    compileSdkVersion safeExtGet('compileSdkVersion', 26)
+    buildToolsVersion safeExtGet('buildToolsVersion', '26.0.2')
 
     defaultConfig {
         minSdkVersion 16