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.

@KingCrunch I don't see any differences: https://getcomposer.org/doc/01-basic-usage.md#next-significant-release-tilde-operator-

afaik the follow constraints all yield the same:
```
  ~5.5
 >=5.5, <6
   5.*, <6
   5.*, >=5.5
```
Regarding the `<6`, I'm not sure how Alpha, Beta and RC are treated...