More TravisCI configuration tweaks.

.travis.yml

@@ -1,8 +1,18 @@
 language: android
 jdk:
   - oraclejdk8
+
 before_install:
+  - mkdir "$ANDROID_HOME/licenses" || true
+  - echo -e "\n8933bad161af4178b1185d1a37fbf41ea5269c55" > "$ANDROID_HOME/licenses/android-sdk-license"
+  - echo -e "\n84831b9409646a918e30573bab4c9c91346d8abd" > "$ANDROID_HOME/licenses/android-sdk-preview-license"
   - chmod +x gradlew
+
+cache:
+  directories:
+    -$HOME/.gradle/caches/
+    -$HOME/.gradle/wrapper/
+
 android:
   components:
     - tools
@@ -10,5 +20,11 @@ android:
     - build-tools-25.0.2
     - android-25
     - extra-google-m2repository
+    - extra-android-m2repository
+  licenses:
+    - android-sdk-preview-license-.+
+    - android-sdk-license-.+
+    - google-gdk-license-.+
+
 script:
-   - ./gradlew build connectedCheck
+   - ./gradlew clean build connectedCheck -PdisablePreDex --stacktrace