Give up `brew update` on Travis

No `brew update` causes "Error: Your Homebrew is outdated" like https://travis-ci.org/ruby/ruby/jobs/547485832,
but doing `brew update` is also problematic like https://travis-ci.org/ruby/ruby/jobs/564916879.

Hoping that the former case is more rare, let's try no `brew update`
again.

.travis.yml

@@ -46,7 +46,10 @@ addons:
       - valgrind
       - zlib1g-dev
   homebrew:
-    update: true
+    # No `brew update` causes "Error: Your Homebrew is outdated" like https://travis-ci.org/ruby/ruby/jobs/547485832,
+    # but doing `brew update` is also problematic like https://travis-ci.org/ruby/ruby/jobs/564916879.
+    # Skipped because `brew update` hangs often
+    # update: true
     packages:
       - gdbm
       - gmp