Use ActiveAdmin HEAD

Issue #1773 was closed, together with the actual fix in #1940. #1940
has been merged, but is not released yet.

The ActiveAdmin in master also contains changed table ids.
Add /print burndown page with print styles applied by default
Merge pull request #33 from kabisaict/fix_active_admin_build

Update to ActiveAdmin HEAD
Merge branch 'master' into feature_burndown_print_view
Link to print-styled page from iterations#show
Open print dialog using JS on iterations#print page.
Merge pull request #35 from kabisaict/feature_burndown_print_view

Add /print burndown page with print styles applied by default
Merge branch 'master' into feature_remove_dead_code