Update required PHP to 5.5.0

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This PR upgrades composer.json files to php>=5.5.0
We can raise the php version later when we'll agree on some higher requirement, but currently, this is required to safelly merge in master without having some users reporting issues because they have a `dev-master@dev` requirement.

Because we somehow know, that PHP7 will (probably) fail, maybe `~5.5` is safer? At some point one can loosen this restriction.