update osx builds to xcode10

Author: gnzlbg

.travis.yml

@@ -38,10 +38,10 @@ matrix:
     - env: TARGET=i686-unknown-linux-gnu
     - os: osx
       env: TARGET=x86_64-apple-darwin NO_ADD=1
-      osx_image: xcode9.4
+      osx_image: xcode10
     - os: osx
       env: TARGET=i686-apple-darwin
-      osx_image: xcode9.4
+      osx_image: xcode10
     - env: TARGET=arm-linux-androideabi
     - env: TARGET=aarch64-linux-android
     # FIXME(#826) should reenable