Bump the open file ulimit on OSX.

Tests showed the default was 256 previously which appears to be too low
for modern pants as evidenced by CI failures caused by too many open
file handles.

Testing Done:
OSX CI ran here:
  https://travis-ci.org/pantsbuild-osx/pants/builds/124555939

That had 1 shard failure in a test, but apparently no open-fh issues
any longer.

That confirms the 256 initial setting and successful bump to 8192.

Still not sure if this will do the trick when it hits master, but
256 is a bit low.

Normal CI cancelled since it does not test the file edited here.

Bugs closed: 3239

Reviewed at https://rbcommons.com/s/twitter/r/3733/

.travis.osx.yml

@@ -45,6 +45,9 @@ script: |
   sw_vers
   python --version
   java -version
+  echo "Bumping open file ulimit from `ulimit -n`..."
+  ulimit -n 8192 # This is probably higher than needed, 1024 limits on linux seem to work fine.
+  echo "...to `ulimit -n`"
   ./build-support/bin/ci.sh -x ${CI_FLAGS}
 
 # The `notifications:` configuration will be programatically copied over from `.travis.yml` for