Update 6.x-or-7.x-to-7.6.md
Created by: NuLL3rr0r
Added missing libkrb5-dev as mentioned here https://github.com/gitlabhq/gitlabhq/blob/master/doc/update/7.5-to-7.6.md
Created by: NuLL3rr0r
Added missing libkrb5-dev as mentioned here https://github.com/gitlabhq/gitlabhq/blob/master/doc/update/7.5-to-7.6.md