contentURL expects a string in Panel() and Widget() constructors

@whimboo [noted][1] ...:
> Looks like memchaser stopped working in the 34.0 builds of Aurora. We should make some progress here to get a compatible version out.

And @tuchida [provided][2] an easy fix, commited here.

@whimboo, Travis CI build failed ... with an unknown reason to me.
Should I add an "Update SDK to latest version" commit here too?



[1]:  https://github.com/mozilla/memchaser/pull/194#issuecomment-54984663
[2]: https://github.com/mozilla/memchaser/pull/194#issuecomment-55098837