Consistently use the same filter for required user capabilities

In redirection.php, line 130:
apply_filters( 'redirection_role', 'administrator' )
is used so that the required capabilities for using 'Redirection' can be modified without modifing the plugin itself.

I think that the same filter should be used in ajax.php
Merge pull request #11 from RodGer-GR/patch-1

Consistently use the same filter for required user capabilities