.travis.yml: configure for OSX

Change-Id: I5096dc5360f3c06c32b39ab07b8ba4f989fa5b0b
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/167377
Reviewed-by: Herbie Ong <herbie@google.com>

.travis.yml

@@ -1,6 +1,16 @@
-dist: xenial
+language: go
+go: 1.12.x
+install:
+  - unset GOROOT
+  - unset GOPATH
+matrix:
+  include:
+    - os: linux
+      dist: xenial
+    - os: osx
+      osx_image: xcode10.1
 script:
   - ./test.bash
 cache:
   directories:
-  - .cache
+    - .cache