Change target SDK to 26 per Google Play recommendation

Change-Id: I5b60c070398ea4c1ee3d3e7dac12a2e62b241311
diff --git a/app/build.gradle b/app/build.gradle
index 3dc9643..dadd233 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -8,8 +8,8 @@
     defaultConfig {
         applicationId "net.named_data.nfd"
         minSdkVersion 23
-        targetSdkVersion 23
-        versionCode 4008
+        targetSdkVersion 26
+        versionCode 4009
         versionName "0.6.2-1 (NFD 0.6.2-11-gd657d53)"
         multiDexEnabled true
     }