There were a few undefined variables being referenced within the
Select2 code. The JSHint configuration has been modified to fail
if there are undefined variables being referenced, including any
global variables.
There is an exception that is being made for the jQuery shim, as it
must try to find the global jQuery references.
An exception is made for the test helpers, as the SauceLabs reporting
variables are not camelCase, and would normally trigger warnings.
This closes https://github.com/select2/select2/pull/3028.
