use $rootScope instead of $scope.$root

[zacronos]

@dhilt unfortunately I just found a problem with my prior PR, which has already made it out into rc.3 on npm.

Apparently, in some circumstances I don't fully understand, $scope.$root can be null, causing an exception in the new $scope.$root.$$phase check. The fix in this PR is to check $rootScope.$$phase instead.

I apologize for the inconvenience, but you may want to consider merging this and then releasing a rc.4 build to get ahead of any errors this will cause.

[dhilt]

@zacronos Of course! as it was her. How do think, is this critical and do we need to release rc.4 immediately? or can it be postponed until next bunch of changes are ready to release?

[zacronos]

@dhilt Since I don't really understand why $scope.$root will ever be null, I don't know how likely this is to cause problems for people -- but it did cause an exception in my project which stopped ui-scroll from working once it occurred, so I would say it's better to treat it as critical and release rc.4 immediately.

[dhilt]

@zacronos Thanks! will do it