chore(package): pin babel version down to 5.4.2 for now
chore(package): pin babel version down to 5.4.2 for now

since babel-plugin-espower does not work with babel-core 5.4.3

falsandtru/power-assert,power-assert-js/power-assert,falsandtru/power-assert,saneyuki/power-assert,saneyuki/power-assert,yagitoshiro/power-assert,power-assert-js/power-assert,twada/power-assert,yagitoshiro/power-assert,twada/power-assert