Starting with version 4.0.0, the library [dot-prop](https://github.com/sindresorhus/dot-prop)
is now [written in
ES2015](sindresorhus/dot-prop@88b6eb6),
which means it doesn't work in IE anymore or when minifying the
code with uglifyjs as the code isn't transpiled.
This commit set es6 as format so that the code is transpiled and
works in IE and when minifying with uglifyjs