Re-enable parallel build/test on OSX

.travis.yml

@@ -362,8 +362,6 @@ env:
       - CONFIG_FLAG=--with-opt-dir=/usr/local/opt/openssl@1.1:/usr/local/opt/zlib
       # Adding `-v` because we're not sure which test could hang forever: https://travis-ci.org/ruby/ruby/jobs/564804923
       - TEST_ALL_OPTS="-v --timeout-scale=2.0 --tty=no --excludes=\$(TESTSDIR)/excludes/_travis/osx"
-      # Disabling -j3 because it seems to cause a hang on building Ruby: https://travis-ci.org/ruby/ruby/jobs/471021727
-      - JOBS=
 
   - &dependency
     name: Check dependencies in makefiles