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.

``5.*, >=5.5`` is rewritten to ``>=5.0, <6-dev, >=5.5``. This will match the same than ``~5.5`` which is rewritten to ``>=5.5, <6-dev`` It will just give more work to Composer when matching constraint by checking 2 lower bound while the 5.5 one is enought to restrict things

``5.*, <6`` is **not** equivalent. It is the same than `5.*`` (given that 5.x is always lower than 6.0), which allows 5.4

And ``>=5.5, <6`` is not equivalent to ``~5.5``, because it uses ``6.0.0.0`` as upper bound for the constraint, which is higher than pre-release for 6.0. The upper bound of the semantic operator is ``6.0.0.0-dev`` (the number of 0 does not matter, but Composer normalizes to 4 digit parts because of version_compare doing crap for different numbers of parts), which is the first dev snapshot of 6.0 (i.e. a tag ``v6.0-dev``, equivalent to what we called "Preview releases" in the Symfony 2.0 history).
Then, if you allow only dev stability, pre-releases will indeed be forbidden. But symfony does not control the stability setting, so it must write its constraint as if the dev stability was allowed to be selected.