TravisCI: Just build x86-64 on OSX.

.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 osx-debug64; fi
+  - if [ "$TRAVIS_OS_NAME" == "osx" ]; then make osx-debug64-x86; fi
 
 branches:
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