Don't shadow outer variables

This guideline is already in effect
in Hound's default .ruby.yml file,
used by Rubocop:

https://github.com/thoughtbot/hound/blob/master/config/style_guides/ruby.yml

It is possible in many programming languages to
name variables the same as the name of
other variables in a different lexical scope.
This is called "variable shadowing."

Shadowing variables can be convenient
but can also cause confusion
and are easy to catch with tools
such as Hound and Syntastic.

http://wizardmode.com/2012/06/shadowing-variables-in-coffeescript/
https://books.google.com/books?id=HW-5SZ1HKusC&lpg=PA189&ots=Fq462aV4rp&dq=shadow%20outer%20variable&pg=PA189#v=onepage&q=shadow%20outer%20variable&f=false
http://rustbyexample.com/variables/scope.html

With `$VERBOSE = true` turned on in Ruby,
the language will print a warning.

https://stackoverflow.com/questions/6259314/what-does-shadowing-mean-in-ruby`