add kerberos dependency

doc/install/installation.md

@@ -54,7 +54,7 @@ up-to-date and install it.
 
 Install the required packages (needed to compile Ruby and native extensions to Ruby gems):
 
-    sudo apt-get install -y build-essential zlib1g-dev libyaml-dev libssl-dev libgdbm-dev libreadline-dev libncurses5-dev libffi-dev curl openssh-server redis-server checkinstall libxml2-dev libxslt-dev libcurl4-openssl-dev libicu-dev logrotate python-docutils pkg-config cmake
+    sudo apt-get install -y build-essential zlib1g-dev libyaml-dev libssl-dev libgdbm-dev libreadline-dev libncurses5-dev libffi-dev curl openssh-server redis-server checkinstall libxml2-dev libxslt-dev libcurl4-openssl-dev libicu-dev logrotate python-docutils pkg-config cmake libkrb5-dev
 
 Make sure you have the right version of Git installed
 

doc/update/7.5-to-7.6.md

@@ -45,6 +45,8 @@ sudo -u git -H git checkout v2.2.0
 ### 4. Install libs, migrations, etc.
 
 ```bash
+sudo apt-get install libkrb5-dev
+
 cd /home/git/gitlab
 
 # MySQL installations (note: the line below states '--without ... postgres')