Drop HHVM support

[Hywan]

After a long discussion about #36:

1. atoum has dropped support for HHVM,
2. No one has asked for HHVM support,
3. HHVM does not support PHP 7.

Progression

• Acl,
• Compiler,
• Consistency,
• Console,
• Database,
• Dispatcher,
• Event,
• Exception,
• Graph,
• Iterator,
• Json,
• Locale,
• Mail,
• Math,
• Mime,
• Protocol,
• Registry,
• Ruler,
• Socket,
• Stream,
• Ustring,
• View,
• Visitor,
• Websocket,
• Zombie.

[Grummfy]

So we remove it totally from travis build?

[vonglasow]

@Grummfy Yes, it was decided during the last virtual meeting.

[Grummfy]

it was just be sure