Bump to xcode10.2 image

Author: fantix

.travis.yml

@@ -29,24 +29,24 @@ matrix:
 
     include:
         - os: osx
-          osx_image: xcode7.3
+          osx_image: xcode10.2
           # Travis macOS env does not support Python yet,
           # so we have to set things up manually in install.sh.
           env: BUILD=tests,wheels PYTHON_VERSION=3.5.5
           branches: {only: [releases]}
 
         - os: osx
-          osx_image: xcode7.3
+          osx_image: xcode10.2
           env: BUILD=tests,wheels PYTHON_VERSION=3.6.5
           branches: {only: [releases]}
 
         - os: osx
-          osx_image: xcode7.3
+          osx_image: xcode10.2
           env: BUILD=tests,wheels PYTHON_VERSION=3.7.0
           branches: {only: [releases]}
 
         - os: osx
-          osx_image: xcode7.3
+          osx_image: xcode10.2
           env: BUILD=tests,wheels PYTHON_VERSION=3.8.0
           branches: {only: [releases]}