[3.0] Update required PHP to 5.5.9

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #13126, #12778
| License       | MIT
| Doc PR        | -

This PR upgrades composer.json files to php>=5.5.9
5.5.9 is the lowest version available on Travis and the version shipped with Ubuntu LTS 14.04.
We can change the php version later when we'll agree on some other requirement, but currently, this is required to safely merge in master without having some users reporting issues because they have a `dev-master@dev` requirement.