Remove HHVM from required packages

[maltsev]

With HHVM as a requirement the the library couldn't be installed using composer require if HHVM isn't installed as well:

> composer require docopt/docopt:1.0.3
Problem 1
  - Installation request for docopt/docopt 1.0.3 -> satisfiable by docopt/docopt[1.0.3].
  - docopt/docopt 1.0.3 requires hhvm >=3.3.0 -> you are running this with PHP and not HHVM.

And if you run just composer require version 1.0.2 would be installed (since it hasn't HHVM in as a requirement).

Related issues: #13, #11.