Update to psr/http-message 0.2.0

- StreamInterface becomes StreamableInterface
- StreamableInterface now defines:
  - attach()
  - getMetadata($key = null)
- StreamableInterface now removes $maxLength argument from getContents()
Updated changelog
Merge branch 'hotfix/update-to-0.2.0'

Close #9
[0.4.0] Marked date in changelog