All URLHandler usage goes through getDefault.

* All URLHandler access goes through the same method
* Removes the reflective lookup of httpclient.

Review by @eed3si9n 
I think we should just have ONE handler. I'd have hardcoded in sbt, but I found Ivy was inconsistent in calling `getDefault` vs. `getHttp` (i.e. the codebase is literred with both calls), so I chose one and stuck with it.


Moving towards async is fine, but beyond the scope of this fix, which is just "use the one we've tested and have proxy support for".