TravisCI Tweak - update bundler before install

The travis CI build for ruby 1.9.3 failed with this error message:

    NoMethodError: undefined method `spec' for nil:NilClass
    An error occurred while installing spark_api (1.3.26), and Bundler
    cannot continue.
    Make sure that `gem install spark_api -v '1.3.26'` succeeds before
    bundling.

Seems like a fair number of people had this problem with ruby 1.9.3 and
bundler 1.7.6, so we'll try the workaround that worked for them.

Author: bhornseth

.travis.yml

@@ -2,3 +2,9 @@ language: ruby
 rvm:
   - 1.9.3
   - 2.1.6
+
+# Try to work around a travisCI quirk
+# see https://git.io/vVf62 for more
+before_install:
+  - gem update --system
+  - gem update bundler