Failure to minify predicate dependency

[donalmurtagh]

This issue was introduced when upgrading from version 0.1.16 to 0.1.17.

After upgrading to 0.1.17, I get this issue when I run npm run build

Failed to minify the code from this file: 
 	./node_modules/predicate/lib/utils.js:7 

The line in question is

  curried.toString = () => src.toString();

Presumably, this fails to minify because it's using an ES6 feature (an arrow function). This is related to issue #23