(maint) Update invoke_task to support passing multiple arguments

Previously the invoke_task method was limited to passing 0 or 1 argument
to the rake task. It is occasionally desireable to pass more than 1
argument, so this commit updates the method to take an arbitrary # of
extra arguments, and pass them on to the Rake::Task invocation.
(maint) Update rm_rf call in file_spec to use FileUtils
(CPR-52) Add deploy_nightly_repos task

This commit adds a deploy_nightly_repos task which will be used to ship
the signed nightly repos to the public nightly repository. It retreives
the repos and repo_configs and then swaps out the distribution server
for the nightly server, to ensure that the repo configs are functional.
This task also creates a latest symlink for the project.
Merge pull request #324 from haus/ticket/master/cpr-52-nightly-repos

(CPR-52) Add deploy_nightly_repos task
(maint) Add ruby 2.1.1 to travis config
(maint) Sort ruby versions in travis config
(CPR-61) Configure travis to run rubocop against PRs

This commit updates the travis config to run rubocop on ruby 2.1.1
against PRs. All other rubies are excluded from rubocop, but will
continue to run specs. This means there will be 5 cells in the matrix, 4
rspec cells, and 1 rubocop cell.
(CPR-61) Pin rubocop version to 0.24.1

Rubocop 0.25.0 introduces several changes that break our current checks,
including moving cops between namespaces, and adding new default cops
that fail on our code. We should move forward to 0.25.0 intentionally
when we are ready.
(CPR-61) Resolve single new rubocop offense in nightly_repos.rake (space after curly brace)