travisci: Limit OSX build only to debug build to prevent CI timeout.

.travis.yml

@@ -24,7 +24,7 @@ before_script:
 
 script:
   - if [ "$TRAVIS_OS_NAME" == "linux" ]; then make build CXX="g++-8" CC="gcc-8"; fi
-  - if [ "$TRAVIS_OS_NAME" == "osx" ]; then make build; fi
+  - if [ "$TRAVIS_OS_NAME" == "osx" ]; then make osx-debug64; fi
 
 branches:
   only:
@@ -33,4 +33,4 @@ branches:
 notifications:
   email: false
 
-osx_image: xcode9.3
+osx_image: xcode10.3