Limit Travis run

Summary:
Right now, Travis is failing consistently because the Android build
time exceeds its limits - that's separate from the new publish step.

Let's limit what we do in the build step.

Reviewed By: jknoxville

Differential Revision: D9613247

fbshipit-source-id: f703237788f3a35d3f15fc8b9b45d6ebbb24c750

.travis.yml

@@ -89,7 +89,7 @@ matrix:
         - export PATH=${PATH}:${ANDROID_NDK_HOME}
         - cd ../
       script:
-        - ./gradlew :sample:build
+        - ./gradlew :sample:assembleDebug
       deploy:
         - provider: script
           script: "scripts/publish-android-snapshot.sh"