Don't change ENV['RAILS_ENV'] in the guard process

We were seeing problems when running our RSpec tests via guard-rspec
after guard-rails had booted Rails.

Now we use the optional first parameter to Kernel#system to set the
environment only for the launched process.
Merge pull request #5 from nycvotes-dev/leave-environment-alone

Don't change ENV['RAILS_ENV'] in the guard process