"engines" in package.json makes installing with yarn trickier

[actionshrimp]

Hi, thanks for the great lib :)

Installing with yarn checks the engines listed in package.json files. If I bypass the check with yarn add --ignore-engines the package installs fine, and seems to be functioning correctly on node v.7.4.0.

Would you mind loosening the restriction or removing it so multimethod-js can be installed more easily without needing to add the flag to bypass the check?

Many thanks