Fixes NullPointerException when there is no latest-artifact available

fixes error that looks like this:
```
~/Projects/kontera/a17-language-detector(master v) lein outdated
[midje/midje "1.6-alpha2"] is available but we use "1.5.1"
[ring-mock/ring-mock "0.1.5"] is available but we use "0.1.4"
java.lang.NullPointerException
        at clojure.lang.Reflector.invokeNoArgInstanceMember(Reflector.java:296)
        at leiningen.outdated$latest_version.invoke(outdated.clj:62)
        at leiningen.outdated$outdated.doInvoke(outdated.clj:74)
        at clojure.lang.RestFn.invoke(RestFn.java:410)
        at clojure.lang.Var.invoke(Var.java:415)
        at clojure.lang.AFn.applyToHelper(AFn.java:161)
        at clojure.lang.Var.applyTo(Var.java:532)
        at clojure.core$apply.invoke(core.clj:619)
        at leiningen.core.main$resolve_task$fn__2160.doInvoke(main.clj:148)
        at clojure.lang.RestFn.invoke(RestFn.java:410)
        at clojure.lang.AFn.applyToHelper(AFn.java:161)
        at clojure.lang.RestFn.applyTo(RestFn.java:132)
        at clojure.lang.AFunction$1.doInvoke(AFunction.java:29)
        at clojure.lang.RestFn.applyTo(RestFn.java:137)
        at clojure.core$apply.invoke(core.clj:619)
        at leiningen.core.main$apply_task.invoke(main.clj:188)
        at leiningen.core.main$resolve_and_apply.invoke(main.clj:192)
        at leiningen.core.main$_main$fn__2223.invoke(main.clj:256)
        at leiningen.core.main$_main.doInvoke(main.clj:246)
        at clojure.lang.RestFn.invoke(RestFn.java:408)
        at clojure.lang.Var.invoke(Var.java:415)
        at clojure.lang.AFn.applyToHelper(AFn.java:161)
        at clojure.lang.Var.applyTo(Var.java:532)
        at clojure.core$apply.invoke(core.clj:617)
        at clojure.main$main_opt.invoke(main.clj:335)
        at clojure.main$main.doInvoke(main.clj:440)
        at clojure.lang.RestFn.invoke(RestFn.java:436)
        at clojure.lang.Var.invoke(Var.java:423)
        at clojure.lang.AFn.applyToHelper(AFn.java:167)
        at clojure.lang.Var.applyTo(Var.java:532)
        at clojure.main.main(main.java:37)
```