Because xunit 1 is just so 2013 😜

Benefits:

• xunit 2.0
  • This will probably get rid the intermittent (9/10) build failures which we see in Travis CI, which I believe is a problem which only manifests in xunit 1.x, on low spec VMs (some kind race condition)
  • parallel test exec
  • self contained VS runner support
  • test collections
  • nicer console output
  • other new xunit 2.0 features - http://xunit.github.io/release-notes/2015-03-16.html
• xbehave 2.0
  • support for all new xunit 2.0 features
  • all steps following a failed step in a scenario are reported as skipped instead of failed, drastically reducing noise in test output
  • other minor changes - https://github.com/xbehave/xbehave.net/wiki/Changes-in-version-2.0
• moving off obsolete platforms which are purely in maintenance mode

Blockers:

• xunit 2.0 currently does not work in Mono, due to a bug in Mono, but this has been fixed and is awaiting release. See xunit 2.0 TypeLoadException on Mono xunit/xunit#158
• AutoFixture.Xunit is still tied to xunit 1.x - see Xunit2 AutoFixture/AutoFixture#294
• xbehave 2.0 is still in RC, but RTM should be in the next week or two - see 2.0 release adamralph/xbehave.net#144