Use default image for osx on Travis.
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

google/jsinterop-base,google/jsinterop-base,google/jsinterop-base