travis: Drop Xcode 8.2

Travis has only so many OSX builders, presumably due to the EULA
requiring that it be run on Apple-branded hardware. They're also really
slow to provision, presumably because they're virtual machines and not
containers. In any case it's not something I'm willing to be punished
for.

Signed-off-by: Adam Jackson <ajax@redhat.com>

.travis.yml

@@ -13,9 +13,6 @@ matrix:
     - os: osx
       osx_image: xcode9.2
       env: DISTRO=xcode9.2
-    - os: osx
-      osx_image: xcode8.3
-      env: DISTRO=xcode8.3
 
 install:
     - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then HOMEBREW_NO_AUTO_UPDATE=1 brew install ccache ; fi