Remove limitation for installing Puppet on trusty, 3.6.0 available from PuppetLabs repo

Hi,

as from 3.6.0 release Puppet became available for Ubuntu Trusty from PuppetLabs repo.

Here I've removed the conditional to print the warning and skipping Puppet installation when building a Trusty based container.

The container contains the following packages:

    facter  2.0.1-1puppetlabs1
    hiera   1.3.2-1puppetlabs1
    puppet  3.6.0-1puppetlabs1
    puppet-common   3.6.0-1puppetlabs1
    puppetlabs-release      1.0-7


As reference: http://docs.puppetlabs.com/puppet/latest/reference/release_notes.html#os-support-changes

Let me know if I've missed anything else that should be modified.