Fix for 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. Changing the conditional on the first line to check for the truthiness of window.customEvent fixes this. This is an alternative fix to #242. Either works. I think this one is more elegant.