Update android sdk in travis

.travis.yml

@@ -5,8 +5,8 @@ android:
     - tools
 
     # The BuildTools version used by your project
-    - build-tools-25.0.1
-    - android-25
+    - build-tools-26.0.1
+    - android-26
     - extra-android-m2repository
     - extra-google-m2repository
     - extra-android-support