require (not contain) yum::plugin::versionlock

[alexjfisher]

The plugin class yum::plugin::versionlock should not be contained
from every instance of the yum::versionlock defined type. Doing so
makes it impossible to use the type in multiple dependent
classes/profiles. contain implies that the resource 'owns' the plugin.
Using require should be just as effective at solving #43 without
introducing dependency cycles.

With contain...

class profile::foo {
  yum::versionlock { '0:bash-4.1.2-9.el6_2.*':}
}

class profile::bar {
  yum::versionlock { '2:vim-enhanced-7.4.629-6.el7.x86_64':}
}

include profile::foo
include profile::bar

Class['profile::foo'] -> Class['profile::bar']

fails with

Error: Found 1 dependency cycle:
(Augeas[yum.conf_plugins] => Yum::Config[plugins] => Yum::Plugin[versionlock] => Class[Yum::Plugin::Versionlock] => Yum::Versionlock[0:bash-4.1.2-9.el6_2.*] => Class[Profile::Foo] => Class[Profile::Bar] => Yum::Versionlock[2:vim-enhanced-7.4.629-6.el7.x86_64] => Class[Yum::Plugin::Versionlock] => Yum::Plugin[versionlock] => Package[yum-plugin-versionlock] => Yum::Plugin[versionlock])\nTry the '--graph' option and opening the resulting '.dot' file in OmniGraffle or GraphViz
Error: Failed to apply catalog: One or more resource dependency cycles detected in graph

This commit also refactors the type with:

• puppet-strings style docs.
• A simple unless instead of a case for ensure.
• assert_type instead of is_a.

Fixes #43