Add missing ext-json in composer.json.

[drupol]

We are using the PHP function json_decode in some places in the code.

Therefore, we need to ensure that users has the proper PHP extension "ext-json".

In order to do that, we need to modify the composer.json and add

"ext-json": "*"

in the require section.

[coveralls]

Coverage remained the same at 38.276% when pulling 50899fb on drupol:add-missing-php-extension into d3b9741 on cweagans:master.

[drupol]

@cweagans Could you review this ?

[drupol]

Thanks !