Use $ACKRC if set, otherwise use $HOME/.ackrc

The $ACKRC environmental variable allows a user to specify a path to his or her Ack config file. If not set, Ack looks for a file in the default location (~/.ackrc).

This update makes ack completion follow that same behavior.
Merge pull request #124 from brandt/master

Use $ACKRC if set, otherwise use $HOME/.ackrc