(PUP-1547) Undefined method for nil:NilClass when no providers found

When a puppet type has a list property and no providers are found for that type,
provider is nil and causes an "Undefined method 'property' for nil:NilClass" error.
ie. Undefined method 'groups' for nil:NilClass for the user type

Check if provider is nil before trying to access the property
(maint) Consolidate redundant join_options implementations.

Moving the implementation of join_options into the base package provider.
This gives a consistent behavior between all package providers that support install_options.
(maint) Remove travis irc notifications

Apologies for all the churn. This removes the irc notifications
because it turns out that travis doesn't (currently) support
notifications for pull request builds, and those are the only
travis builds we really care about (since we have more comprehensive
jenkins builds once pulls are merged).
Merge pull request #2498 from peterhuene/maint/install_options

(maint) Consolidate redundant join_options implementations.
Merge pull request #2311 from carlossg/PUP-1547

(PUP-1547) Undefined method 'groups' for nil:NilClass when no providers found
Merge branch 'master' of https://github.com/puppetlabs/puppet