ignite_core: Allow Linkit 7.x to be used

Problem/Motivation

linkit 7.0.0-alpha1 is available for installation.

https://www.drupal.org/project/linkit/releases/7.0.0-alpha1
"...there are no identified issues blocking a stable release. When the maintainers see sufficient adoption of the 7.x branch, a stable release will be provided."

Proposed resolution

Allow Linkit 6.x or 7.x for installation.

Note that 7.x does "Remove CKEditor 4 support".