FOR REVIEW: (SERVER-71) Add support for configuring http client ssl protocol

This currently has a dep on a SNAPSHOT of clj-http-client, so shouldn't be merged yet.
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This commit does the following:

* Upgrade us to the latest version of clj-http-client, which includes
  a default list of legal ssl-protocols and makes them configurable
* Add a new `http-client` settings section, in which you can configure
  the legal ssl-protocols and cipher suites for any http client requests
  that are issued by Puppet Server
Not worth changing but just FYI you can do `(is false "Failure message")` to force the failure.  Kinda wish there was a `(fail "Failure message")` function in clojure-test, or a better way to do this without it looking weird.