Update continuous-integration docs with sudo: false information

Updates the continuous-integration docs to provide information about the `sudo: false` behavior which can route builds to the faster container-based infrastructure. https://docs.travis-ci.com/user/workers/container-based-infrastructure/#Routing-your-build-to-container-based-infrastructure

site/_docs/continuous-integration.md

@@ -112,6 +112,8 @@ branches:
 env:
   global:
   - NOKOGIRI_USE_SYSTEM_LIBRARIES=true # speeds up installation of html-proofer
+
+sudo: false # route your build to the container-based infrastructure for a faster build
 {% endhighlight %}
 
 Ok, now for an explanation of each line:
@@ -202,6 +204,16 @@ environment variable `NOKOGIRI_USE_SYSTEM_LIBRARIES` to `true`.
 exclude: [vendor]
 {% endhighlight %}
 
+By default you should supply the `sudo: false` command to Travis. This command 
+explicitly tells Travis to run your build on Travis's [container-based
+ infrastructure](https://docs.travis-ci.com/user/workers/container-based-infrastructure/#Routing-your-build-to-container-based-infrastructure). Running on the container-based infrastructure can often times
+speed up your build. If you have any trouble with your build, or if your build
+does need `sudo` access, modify the line to `sudo: required`.
+
+{% highlight yaml %}
+sudo: false
+{% endhighlight %}
+
 ### Troubleshooting
 
 **Travis error:** *"You are trying to install in deployment mode after changing