(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)
Merge pull request #325 from haus/ticket/master/cpr_61_enable_rubocop_for_travis

(CPR-61) Configure travis to run rubocop against PRs
Merge pull request #326 from haus/maint/master/add_more_wget_recursion

(maint) Add -l to the wget call in retrieve
(maint) Make ignore-existing optional for Pkg::Util::Net.rsync_to

There are occasions where it is desirable for rsync_to to clobber
existing files, such as for updating a latest directory on a nightly
repository. Previously, this behavior wasn't possible without shelling
out to a custom rsync, so this commit adds a parameter to the rsync_to
method to allow ignore-existing to be excluded from the rsync flags. It
defaults to true, to maintain existing behavior as the default.
(CPR-64) Refactor latest linking for nightly repos

This commit updates the nightly repos workflow to maintain a static
$project-latest directory with a linked repos directory. The
repo_configs are generated to have static filenames and contents, to
ensure that users can simply add the repos and have their package
manager keep them up to date with $project's master branch. The repos
symlink in the latest directory is updated for each project when a new
version ships, and this should be totally transparent to users of the
latest symlink.
This also tweaks the rsync out to not use --ignore-existing, as
otherwise the -latest directory is never updated or replaced.