When enforce_available_locales is enabled only set available locales

[pacoguzman]

This PR implements #14

When the enforce_available_locales flag is enabled, which is the default setting a locale that is not in the list of available locales an InvalidLocale exception is raised. With this changes we're going to set only available locales when is enabled, and any locale when it's not.

A slightly difference with the proposal code from the issue is that we're comparing the full language codes without fallback. So I'm not sure if this PR is too restricted in this way

[mpalmer]

Hi @pacoguzman, thanks for doing this work. Having this code in a PR will make it much easier to work with and get it merged. I'll review it over the next few days and get back to you with comments.

One thing I notice is that the CI runs are failing, on all of the "minimum versions" gemfiles. I'd appreciate it if you could fix that up (I expect a gem just needs a bump in minimum version, both in the test gemfile and also a corresponding change in the gemspec).

[pacoguzman]

Oh sorry I didn't check that, I'll try to solve that failure.

[pacoguzman]

I fixed the specs on I18n 0.4.0 that method is not defined, so I just check if i18n responds to that method

[mpalmer]

I'd prefer it if the behaviour of the middleware didn't depend on which version of a gem was installed, especially if that behaviour difference isn't very well-documented. Please determine which version of i18n introduced the method, and bump the dependencies to that version.

[pacoguzman]

Hi Matt I bump the dependencies to the first one where that method is defined, please take a look

[zedtux]

@mpalmer can you please review this PR and merge it?