Add a package for [ack](http://beyondgrep.com/install/). Simply install
the fatpacked script.
It uses '#!/usr/bin/env perl' and it very much not choosy about what
perl it needs. For now just trust that there's one available, perhaps
someday we can/should uncomment the depends_on('perl').
Follows the methodolgy I used in nextflow. Has the same
uninstall/install problem that nextflow has, there is an issue in
progress for that: https://github.com/LLNL/spack/issues/1308.
Tested on CentOS7.
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