Fixed illegal return in FastClick.js

When using `browserify` with `deamdify`, FastClick can break any browserify transform that uses Esprima because FastClick essentially has an illegal return. i.e. without the AMD wrapper, there is a return statement at the Program level scope. This is an illegal return because ECMA-262 does not allow you to return at the global/Program level scope. Wrapping FastClick in an IIFE fixes this, although inverting the first conditional with the illegal return to wrap the entire body of FastClick should work as well. If this other fix is preferable, I can close this pull request and submit it again with that solution.