v0.2.3

Update notes:

• Newer versions of Pretender will show a warning
  if you forget to shut down a pretender server before starting another one. Mirage does this
  for you in acceptance tests, but if you are starting Mirage manually in other tests, remember
  to shut down the server when you are done.
  (see #925)

Changes:

• [FEATURE] Add support for promises in route functions
  (#924)
  @dustinfarris
• [BUGFIX] Adjust afterCreate callbacks execution order
  (#893)
  @Azdaroth
• [INTERNAL] Shutdown pretender servers in unit tests
  (#917)
  @dustinfarris