Bump to xcode8 to avoid rust-lang/regex#489

Author: stuhood

.travis.yml

@@ -63,10 +63,9 @@ matrix:
   include:
     - os: osx
       # We request the oldest image we can (corresponding to OSX 10.11) for maximum compatibility.
-      # We use 10.11 not 10.10 because 10.11 has a functioning version of lldb which can be useful
-      # for debugging native code issues.
+      # We use 10.11 as a minimum to avoid https://github.com/rust-lang/regex/issues/489.
       # See: https://docs.travis-ci.com/user/reference/osx/#OS-X-Version
-      osx_image: xcode7.3
+      osx_image: xcode8
       stage: Test Pants
       language: generic
       env: