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.

Personally I prefer verbosity: `"5.*, >= 5.5"`. But afaik, `~` is standard within symfony. When php7 is about to be released, it's a good idea to put php7 in travis and have phpunit test against it. Until than, no compatibility can be guaranteed with php7, thus it should be within the 5.* range.