Update to ex_cldr 2.14+, fix tests, docs

[kipcole9]

Enhancements

• Requires ex_cldr version 2.14 or later to use CLDR data version 37. As with all CLDR releases, underlying data can change and tests are updated to reflect those changes.

• Updated readme to reflect changes in EU composition and the updated data returned territories

• CLDR 37 also reflects that the UK is no longer part of the EU.

• Minimum version of Elixir required is 1.8 to align with the baseline ex_cldr library

• Fix typespec leading to dialyzer errors

• Update dependencies in development

[kipcole9]

@Schultzer, the current version on hex is incompatible with the current line of ex_cldr since 2.14 so I've updated everything to come back to alignment. Hopefully its an easy PR to accept and then publish to hex.