Try disabling Travis cache on darwin

to check if it resolves "Permission denied" problem.
https://travis-ci.org/ruby/ruby/jobs/507381339
https://travis-ci.org/ruby/ruby/jobs/507392198

TBH I'm not exactly sure why it happens, but I don't have infinite time
to understand this. It would be appreciated to identify the root cause
and remove `DISABLE_CACHE` again if you're sure your patch fixes the problem.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@67281 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

.travis.yml

@@ -278,6 +278,10 @@ env:
       - TEST_ALL_OPTS="--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=
+      # Disabling cache because x86_64-darwin build randomly fails by "Permission denied" which we don't know why:
+      #   https://travis-ci.org/ruby/ruby/jobs/507381339
+      #   https://travis-ci.org/ruby/ruby/jobs/507392198
+      - DISABLE_CACHE=1
 
   - &universal-darwin17
     name: uinversal.x86_64h-darwin17
@@ -318,6 +322,10 @@ before_script:
   - $SETARCH uname -r
   - rm -fr .ext autom4te.cache
   - echo $TERM
+  - |-
+    if [ -n $DISABLE_CACHE ]; then
+      rm -fr ~/.downloaded-cache ~/config_2nd
+    fi
   - |-
     [ -d ~/.downloaded-cache ] ||
     mkdir ~/.downloaded-cache