Fixed Buffer Deprecation warning (#560)

I've been seeing a warning for `DeprecationWarning: Buffer() is deprecated due to security and usability issues.`. This fixes that.

src/functions.js

@@ -581,7 +581,7 @@ const functions = (() => {
  // Simply doing `new Buffer` at this point causes Browserify to pull
  // in the entire Buffer browser library, which is large and unnecessary.
  // Using `global.Buffer` defeats this.
- return new global.Buffer(str, 'base64').toString('binary');
+ return new global.Buffer.from(str, 'base64').toString('binary'); // eslint-disable-line new-cap
  };
  return atob(str);
  }