Fixes linkPurpose.linkHasMoreThan4Chars issue

Fixes issue globant-ui/arialinter#15 where link should have At Least 4 Characters, not more than 4.

Match the rule as stated in http://www.openajax.org/member/wiki/Accessibility_-_WCAG20_Validation_Rules.  Including Rule URL in the definition.  Updating message text to be "4 or more" characters versus "more than 4"