(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