uses humane-test-output to pretty print test outputs

[mcorbin]

I just discovered humane-test-output.
Before using it :

FAIL in (foo-test) (riemann.config:25)
expected: (= (:foo result) [{:host "localhost", :service "foo1", :metric 10}])
  actual: (not (= [{:host "localhost", :service "foo", :metric 10}] [{:host "localhost", :service "foo1", :metric 10}]))

after :

FAIL in (foo-test) (riemann.config:25)
expected: [{:host "localhost", :service "foo", :metric 10}]
  actual: [{:host "localhost", :service "foo1", :metric 10}]
    diff: - [{:service "foo"}]
          + [{:service "foo1"}]

this PR fix #787.

[jamtur01]

LGTM

[jamtur01]

Thanks!