Ensure ntp is installed, not latest

Fixes GH-59, which states:

This is a very quick way to an unstable system and prevents controlling
when updates are conducted.

An example scenario is when new packages make it to the repos but they
config requirements have also changed. Anyone using this module will
have their origin install begin to fail on the next puppet run until
they figure out they need to update the module and only then would their
install start working again.

Another case is where a system under use probably needs to be update
during off hours to allow maximum uptime durring the day.

Instead change all package resources to read ensure => present which
allows the administrator to update the module and the packages during a
planned window.
Merge pull request #216 from ekohl/ensure-ntp-installed

Ensure ntp is installed, not latest