[Checkins] [zopefoundation/zope.index] b1e8db: use marker and set btree minimum requirement

[GitHub]

  Branch: refs/heads/none_value_field
  Home:   https://github.com/zopefoundation/zope.index
  Commit: b1e8db3a5eac91e24e615aa1a7628eb45cc375f5
      https://github.com/zopefoundation/zope.index/commit/b1e8db3a5eac91e24e615aa1a7628eb45cc375f5
  Author: Forest Gregg <fgregg at uchicago.edu>
  Date:   2017-04-21 (Fri, 21 Apr 2017)

  Changed paths:
    M setup.py
    M src/zope/index/field/index.py

  Log Message:
  -----------
  use marker and set btree minimum requirement