Add libyaml-dev to the apt-get install string.

If libyaml headers are not installed, YAML will not be correctly compiled (either not at all, or using Syck instead of Psych).

doc/installation.md

@@ -55,7 +55,7 @@ The installation consists of 6 steps:
     sudo apt-get update
     sudo apt-get upgrade
 
-    sudo apt-get install -y git-core wget curl gcc checkinstall libxml2-dev libxslt-dev sqlite3 libsqlite3-dev libcurl4-openssl-dev libreadline-dev libc6-dev libssl-dev libmysql++-dev make build-essential zlib1g-dev libicu-dev redis-server openssh-server git-core python-dev python-pip sendmail
+    sudo apt-get install -y git-core wget curl gcc checkinstall libxml2-dev libxslt-dev sqlite3 libsqlite3-dev libcurl4-openssl-dev libreadline-dev libc6-dev libssl-dev libmysql++-dev make build-essential zlib1g-dev libicu-dev redis-server openssh-server git-core python-dev python-pip libyaml-dev sendmail
 
 # 2. Install ruby