Use default image for osx on Travis.

xcode8 (macOS: 10.11, jdk:1.8.0_92-b14) is too old and is not compatible with Bazel 2.0.0+. The default config uses Xcode 9.4.1, macOS 10.13 and jdk 10.0.1+10 and works with Bazel 2.0.

PiperOrigin-RevId: 308727886

Author: jDramaix

.travis.yml

@@ -1,7 +1,6 @@
 dist: xenial
 sudo: required
-# xcode8 has jdk8
-osx_image: xcode8
+
 # Not technically required but suppresses 'Ruby' in Job status message.
 language: java