Update gemspec to work with Rails 4.1

- Changed the dependencies to use >= rather than ~> for all Rails
  specific Gems in order to run Shoppe with a Rails 4.1 app.

- Update 'ransack' gem dependency for Rails 4.1
Update to fix issue with Ruby 2.1.1

Removing two return values that were being used in blocks giving
(LocalJumpError: unexpected return) errors from Ruby and causing the
tests to fail.
Rename scope :public to :publicly_visible

As of
https://github.com/rails/rails/commit/815c9deae8de557688e1c99aabe30454bcbe5970
scopes can no longer be named 'public', therefore the 'public' scope is
now called 'publicly_visible'.