.travis.yml: avoid using -j3 for osx

osx build has failed too often. we don't know why.

To know which test hangs forever, we might want to have hard timeout for
all test case (like 9min, slightly less than no-output timeout) in test-all.
But it's a little hard to implement and I would workaround an unknown
cause somehow.

It's known that -j is harmful for some tests in test-all. Let's try to
remove it first, and if it doesn't work, I'll resurrect -v option to
easily know which test is the culprit.

Slow correctness is much better than fast false-positive.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66482 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

.travis.yml

@@ -279,7 +279,8 @@ env:
     <<: *osx
     env:
       - CONFIG_FLAG=--with-opt-dir=/usr/local/opt/openssl@1.1:/usr/local/opt/zlib
-      - TEST_ALL_OPTS="$JOBS -q --tty=no --excludes=\$(TESTSDIR)/excludes/_travis/osx"
+      # Not using $JOBS which might be harmful for some tests because osx build often hangs for unknown reasons.
+      - TEST_ALL_OPTS="-q --tty=no --excludes=\$(TESTSDIR)/excludes/_travis/osx"
 
   - &universal-darwin17
     name: uinversal.x86_64h-darwin17