flake8: hardcode version and dependencies

Fix flake8 upgrades introducing new style errors. See, e.g.,

https://travis-ci.org/uwescience/datalogcompiler/builds/21629661
Add operator id, make operator name human readable
rename opID to opId
Merge pull request #163 from uwescience/hardcode-flake8-version

flake8: hardcode version and dependencies
Merge pull request #169 from uwescience/domoritz-json-format

Change JSON format
Merge branch 'master' into domoritz-min-max
expressions_library: refactor with reduce

See https://github.com/uwescience/datalogcompiler/pull/164#commitcomment-5986637
Merge pull request #164 from uwescience/domoritz-min-max

Add min (least) and max (greatest) expressions
Merge branch 'master' into shumo_add_string_functions