3.1.0-alpha1

• Now requires PHP 5.5
• Upgraded to vobject 4, which is a lot faster.
• Support for PHP 7.
• #690: Support for calendar-availability, draft 05.
  reference.
• #691: Workaround for broken Windows Phone client.
• The zip release ships with sabre/vobject 4.0.0-alpha1,
  sabre/http 4.0.0, sabre/event 2.0.2,
  sabre/uri 1.0.1 and sabre/xml 1.1.0.