3.1.1

• #755: The brower plugin and some operations would break when scheduling and
  delegation would both be enabled.
• #757: A bunch of unittest improvements (@jakobsack).
• The zip release ships with sabre/vobject 4.0.1,
  sabre/http 4.2.1, sabre/event 3.0.0,
  sabre/uri 1.0.1 and sabre/xml 1.3.0.