(maint) Set Travis CI to run two spec runner processes.

Setting Travis CI to use two concurrent processes for running specs.
This should hopefully speed up running specs for pull request CI.

.travis.yml

@@ -1,6 +1,6 @@
 language: ruby
 bundler_args: --without development
-script: "bundle exec rake \"parallel:spec[1]\""
+script: "bundle exec rake \"parallel:spec[2]\""
 notifications:
   email: false
 rvm: