Doc for update from 5.3 to 5.4
Update installation.md

fixed branch-name (5-3 -> 5-4)
Merge pull request #4589 from lukas2511/patch-1

Update installation.md
Update VERSION to 5.4.0
Update references to 5-3-stable in install guide
Merge branch '5-4-stable' of github.com:gitlabhq/gitlabhq

Conflicts:
	VERSION
	doc/update/5.3-to-5.4.md