(#1391) Correct Ubuntu Trusty mod_prefork package name

This module calls apache2-prefork-dev as the package
name for the mod_prefork module for all Debian-flavored Linuxes. However
Ubuntu 14.04 changes this to apache2-dev and makes apache2-prefork-dev
an alias.

The result is that calling this resource, even with the package
installed, always results in a package changed purged to present
message, as it's scanning dpkg for apache2-prefork-dev, finding the
package missing, and then installing the alias via apt.
@eshamow i would rather use [versioncmp()](https://docs.puppetlabs.com/references/latest/function.html#versioncmp), as this is also [the same in 13.10](http://packages.ubuntu.com/search?keywords=apache2-dev) (and 14.10++)