[3.0] Update required PHP to 5.5.9
[3.0] Removed some old hacks
minor #12522 [3.0] Update required PHP to 5.5.9 (nicolas-grekas, GrahamCampbell)

This PR was merged into the 3.0-dev branch.

Discussion
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[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.

Commits
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35e0845 [3.0] Removed some old hacks
fddcb86 [3.0] Update required PHP to 5.5.9