Update Gitlab-shell to newest

newest version of gitlab-shell avail. (has bug fix)

 Clear redis cache and precompile assets during update to 6.0. Fix #4874 #4840

As a consequence on the namespaces change from 5.4 to 6.0, after upgrade
lots of links on the dashboard were not working (pointing to projects
without the group part on the url).
Also, after upgrading to 6.0-stable, the network (branch history) function
does not work. The progress bar rotates forever, but the network graph cannot
be generated.

Clearing redis cache fixes those issues.

Update 20130419190306_allow_merges_for_forks.rb

Postgress db migrate fails.(5.4=>6.0)

More fixes for supporting Relative URL

Make flash messages fixed and sticked to bottom