Downgrade OSX images to xcode 8.3

This mirrors 27d86135eae1d3d38079e7d9f449100251745be2 in deeptest; it reverts OSX builds to using
a Java 8 environment, where the sun.* packages have not yet been removed. Obviously this is not a
long-term fix, but let's leave it to Deeptest to sort this out.

Author: smowton

.travis.yml

@@ -87,6 +87,8 @@ jobs:
     # OS X using g++
     - stage: Test different OS/CXX/Flags
       os: osx
+      jdk: openjdk8
+      osx_image: xcode8.3
       sudo: false
       compiler: gcc
       cache: ccache
@@ -105,6 +107,8 @@ jobs:
     # OS X using clang++
     - stage: Test different OS/CXX/Flags
       os: osx
+      jdk: openjdk8
+      osx_image: xcode8.3
       sudo: false
       compiler: clang
       cache: ccache
@@ -251,4 +255,3 @@ notifications:
     on_start: never
     on_cancel: never
     on_error: always
-