update .travis.yml & move screenshots to separate folder

.travis.yml

@@ -2,7 +2,7 @@ language: android
 android:
   components:
     # The BuildTools version used by NewPipe
-    - build-tools-23.0.1
+    - build-tools-23.0.2
 
     # The SDK version used to compile NewPipe
     - android-23

README.md

@@ -13,8 +13,8 @@ Project status:
 
 ## Screenshots
 
-[<img src="assets/screenshot_1.png" width=200>](assets/screenshot_1.png)
-[<img src="assets/screenshot_2.png" width=200>](assets/screenshot_2.png)
+[<img src="screenshots/screenshot_1.png" width=150>](screenshots/screenshot_1.png)
+[<img src="screenshots/screenshot_2.png" width=150>](screenshots/screenshot_2.png)
 
 ## Description