(maint) Make rails gems optional for rake tasks

Previously running rake -T without certain gems installed would trigger an
error such as "Could not find test-unit (= 1.2.3) amongst [facter-1.6.16,
json-1.7.5, json2graphite-0.0.7, munin-ruby-0.2.3, netaddr-1.5.0,
yam2g-0.0.1]". This gem (and others), shouldn't be necessary to run the
rakefile, list the tasks, or build packages. This commit addresses that by
iterating over the troublesome requires, rescuing any LoadErrors, and then
printing a message if the dependency could not be loaded.
Merge pull request #179 from haus/make_depends_optional_in_rakefile

(maint) Make rails gems optional for rake tasks
[packaging] remove natty distribution from builds

This commit removes Ubuntu natty from the list of distributions to build for,
as its EOL and we no longer build for it.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
(maint) Improve message during a load error in Rakefile

If the loading fails on "tasks/rails", it is generally because of a dependency
in one of the tasks. The current messaging won't give details of which require
failed in the task. This commit adds the message from the LoadError, which
gives details about which require failed.
Merge pull request #180 from MosesMendoza/maint/1.2rc/remove_natty_cow

[packaging] remove natty distribution from builds
Merge pull request #181 from haus/improve_rakefile_messaging

(maint) Improve message during a load error in Rakefile
(packaging) Update VERSION to 1.2.23-rc2