Install libyaml on travis to possibly fix error.

.travis.yml

@@ -22,7 +22,7 @@ before_install: # Use this to prepare the system to install prerequisites or dep
 install:
   - sudo apt-get update
   # Install newer version of ruby
-  - sudo apt-get -qq install software-properties-common
+  - sudo apt-get -qq install libyaml-dev software-properties-common
   - sudo apt-add-repository -y ppa:brightbox/ruby-ng
   - sudo apt-get update
   - sudo apt-get -qq install ruby2.2-dev ruby2.2
@@ -41,4 +41,4 @@ script:
   # Test build with ROS-version of Sphinx command so that it is generated same as ros.org
   - rosdoc_lite -o build .
   # Run HTML tests on generated build output to check for 404 errors, etc
-  - htmlproofer ./build --only-4xx --check-html --file-ignore ./build/html/genindex.html,./build/html/search.html,./build/html/index-msg.html --alt-ignore '/.*/' --url-ignore '#'
+  - htmlproofer ./build --only-4xx --check-html --file-ignore ./build/html/genindex.html,./build/html/search.html,./build/html/index-msg.html --alt-ignore '/.*/' --url-ignore '#'