Project#default_branch use repo HEAD instead of DB value now

Drop default_branch field from projects table
Use repository.root_ref as value for defautl_branch method
Fixes issue with default_branch and HEAD getting out of sync

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Replace assign with stub for default_branch

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Remove default_branch support from API project creation

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Fix spinach tests

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
update CHANGELOG with default_branch changes

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'improve/default_branch' of /home/git/repositories/gitlab/gitlabhq