Drop support for PHP 5.5

[xiehan]

We were supporting it initially because not all of our local machines and VMs were on PHP 5.6 yet, but that is no longer the case, and PHP 5.5 is EOL, so we should remove it.

A few places it should be removed (there might be others; please double-check):

• composer.json
• .travis.yml
• README.md