Update installation.md

Add the -y option to the python apt-get install commands to be consistent with the rest of the Document.
Update merge requests from fork project to origin project

Fix error described in https://github.com/gitlabhq/gitlabhq/issues/5185
Merge pull request #5290 from zzet/patch-3

Update merge requests from fork project to origin project
Merge pull request #5278 from ZaaLabs/master

Small consistency addition for installation.md