(TK-23) Update to clj-http-client 0.2.0

This commit updates jetty9 to the latest clj-http-client release,
and in the process, fixes some transient test failures having
to do with a race condition.  (SSL-related tests that were intended
to cause exceptions could end up with one of two different exceptions
when we use an asynchronous HTTP client.)
I was thinking it might be nice to have what happens in the "false" case be propagated up into the test failure somehow -- e.g., by just doing the asserts inside the macro or returning a complex result back with the failure reason -- so that when a test fails the reason would be easier to spot.  Where these tests have had some weird timing issues that have made it difficult to reproduce test failures, having the failure reason available for a given test run could help us shore this up later.  That said, the implementation as it stands is pretty simple, which is good.