Update bundler before install

* Avoids NoMethodError: undefined method `spec' for nil:NilClass
* Similar approach used in thoughtbot/factory_bot@3c0547f

Author: rgarner

.travis.yml

@@ -12,6 +12,10 @@ matrix:
     - rvm: ruby-head
     - rvm: jruby
 
+before_install:
+  - gem update --system
+  - gem update bundler
+
 bundler_args: --without guard docs
 
 script: bundle exec rspec spec/ -b