CollectionItem will hold functionality that's specific to items
belonging to a collection. Initially, this covers the retrieval of the
value depending on what's passed into the :text_method, :value_method or
:hint_method params.

These arguments now accept and properly deal with procs

Refs #68

Instead of using `#send` inline to retrieve the value from `@item`, make
use of `#retrieve` which works with procs.

Refs #68

The :value_method, :text_method and :hint_method params passed to radio
collections and the :hint_method params passed to checkbox collections
now accept procs which can be used to customise the value retrieved from
the item.

This could be used for simple transformations like upcasing the text or
more-complex operations like localising the value, as per the provided
specs

Refs #68

Some builds are failing before the lint/tests are run (when installing
and compiling gems). Try temporarily disabling the cache