Travis: Move build commands to proprietary script

Prevents an implied attempt to sign builds without a signing key when submitting to a PR.

.travis.yml

@@ -47,7 +47,7 @@ script:
 - git fetch --unshallow
 - cd shell/android-studio
 - export NUMBER_OF_PROCESSORS=2
-- "./gradlew build"
+- "./travis-build.sh"
 - test -z "$encrypted_c726d225a9d9_key" || ./travis-release.sh
 before_deploy:
 - cd ../../

shell/android-studio/travis-build.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+set -ev
+if [ "${TRAVIS_PULL_REQUEST}" = "false" ]; then
+	./gradlew build
+else
+	./gradlew assembleDebug
+fi