Switch to using salt pecl state

Salt now support PECL natively, so use that for package mgmt:

http://docs.saltstack.com/en/latest/ref/states/all/salt.states.pecl.html
Remove dead code: pecl/init.sls

There was no active code in here, remove rather than rely on
comments.
Update CHANGELOG
Merge pull request #4 from ministryofjustice/use_pecl_state

Use state.pecl rather than state.cmd to call out to PHP package mgmt.