Implement JsonSerializable interface

[Daanvm]

Currently encoding an Enum object with json_encode() returns {}, which isn't really useful.

This has been requested before in #30 and #60, but the JsonSerializable interface only exists since PHP 5.4. This library still supports PHP 5.3.

However, now we're a few years later, PHP 5.3 is end of life for over 3 and a half years. The current build doesn't even succeed because PHP 5.3 is no longer available in Travis. I think supporting such old versions of PHP isn't a valid argument anymore.

I am more than willing to create a pull request if you would allow me to bump the PHP requirement to 5.4.

[willemstuursma]

Note that if a new version for PHP 5.4 is created, Composer will still install older versions for PHP 5.3 users and prevent the incompatible version from being installed.

[mnapoli]

I agree, it is time to drop PHP 5.3. I had a look at packages that use PhpEnum (https://packagist.org/packages/myclabs/php-enum/dependents) and I couldn't see any big one compatible with PHP 5.3.

@Daanvm I'm 👍 with a PR that drops PHP 5.3 :) Then we'll be able to support serialization.

[Daanvm]

Great! I'll create a PR for dropping PHP 5.3 and another PR to add JSON serialization support.

[Daanvm]

I've created PR #68 to drop support for PHP 5.3 and another PR #69 that implements the JsonSerializable interface.