Allow a single argument, rather than an array
(maint) refactor ensure_packages for clarity
(maint) Update ensure_package specs to confirm expected behavior

The previous behavior of the tests checked the behavior of the
underlying functions library when called with no arguments; this commit
updates the tests to conform to the functions API and test what happens
when a function is called with no args.
Merge branch 'pull-201'

This closes GH-201.