Use "location.assign" instead of "location ="

As it currently stands, the stable release of closure-compiler errors out due
to the use of "location = value;".

See: 
https://groups.google.com/forum/#!topic/closure-compiler-discuss/sbAQr18Xe9g

According to https://developer.mozilla.org/en-US/docs/Web/API/window.location,
`window.location` is technically read-only but lets you assign to it using `=`
anyway, to change the URL.

Because of that, this is definitely a closure-compiler issue (and was fixed in
git), but it seems like a simple enough change that can be made in
purescript-simple-dom so that the stable closure-compiler can work with it
too.

There is no functionality change at all, this simply uses `.assign()` instead
of `=` and according to the specification, they do the same thing.

Signed-off-by: Ricky Elrod <ricky@elrod.me>