Enable macOS (a.k.a. OS X) Travis CI testing for Gradle builds

We're not going to attempt macOS Travis CI testing for Maven builds,
because I don't know whether that's even expected to work on the
official WALA master branch.  Our main focus here is Gradle.

Note that Travis macOS images do not support JDK switching, so
explicitly selecting the JDK version now becomes part of the
Linux-only configuration.

Travis macOS images also do not support Android as a build "language".
So our Travis CI configuration for Gradle builds now declares that
this is a Java project rather than an Android one.  That's OK, though,
because our Gradle scripts already handle downloading the Android SDK;
we don't need Travis CI to do that for us.  When building using Maven,
though, we still call this an Android project because Maven builds do
still rely on Travis CI to provide the Android SDK.

squash! Enable macOS (a.k.a. OS X) Travis CI testing for Gradle builds

.travis.yml

@@ -1,9 +1,7 @@
 sudo: false
-language: android
 android:
   components:
     - build-tools-26.0.2
-jdk: oraclejdk8
 addons:
   apt:
     packages:
@@ -24,6 +22,16 @@ env:
   global:
     - secure: KcugjQYnBqeZ7XenZD5QY7jhekVPO0QpQyjDUteLytaokhyRK2g6eNvr/pPerN2uWUvsPwO18P9F+oOupge1cpPZf4cEY8RzLIromyUoRWd6JA0SKciUYdN2kSqnC4uZSJGXeGLoroyEEL4Q2sqimpkbIGxgxYtVniWgJULOyR4=
     - secure: jacvGPYX4ugG/HgwJEEpWnllWsS/ipX+qRJ7qM5mbDYryeqsz0eiaxBxQ6IpDyj0v6O4DUi74lSQs/UxCWcUxvOn/5AweCZfoF1U8vt1xivanR4MbC2kr6rJ6ohICuJ4PHDS4IffncgaP3Y8cfExHq6+d0tbibYkjRIiMUGpSik=
-  matrix:
-    - BUILD_SYSTEM=gradle
-    - BUILD_SYSTEM=maven
+matrix:
+  include:
+    - os: linux
+      env: BUILD_SYSTEM=gradle
+      language: java
+      jdk: oraclejdk8
+    - os: linux
+      env: BUILD_SYSTEM=maven
+      language: android
+      jdk: oraclejdk8
+    - os: osx
+      env: BUILD_SYSTEM=gradle
+      language: java