latest process-pool

babel preset changes: es2015-loose -> es2015, stage-1 -> stage-2

update minimum sigh-babel/sigh-mocha dependency versions, remove moch…

…a/babel-core dev dependencies

merge's collectInitial option was broken

support plugins built with babel 6

update engines and fs-extra

version 0.12.29

handle errors thrown by plugins much better, closes #43

release 0.12.27

version 0.12.26