Upgrade gradle

Change-Id: I1a899e2e757c1266a60afdb4e43d9b2ab9805639
diff --git a/app/build.gradle b/app/build.gradle
index ab592cb..b06138c 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -3,8 +3,8 @@
 apply plugin: 'com.android.application'
 
 android {
-    compileSdkVersion 23
-    buildToolsVersion "23.0.2"
+    compileSdkVersion 26
+    buildToolsVersion "26.0.0"
 
     defaultConfig {
         applicationId "net.named_data.nfd"