Update buildout

[mfussenegger]

Build failed otherwise with

Couldn't find index page for 'zope.interface' (maybe misspelled?)
Getting distribution for 'zope.interface==4.1.1'.

[coveralls]

Coverage remained the same at 83.982% when pulling 2c24acc on j/update-buildout into cdde761 on master.

[codecov-io]

Codecov Report

Merging #239 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #239   +/-   ##
=======================================
  Coverage   83.98%   83.98%           
=======================================
  Files          19       19           
  Lines        2235     2235           
=======================================
  Hits         1877     1877           
  Misses        358      358

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