Command line --checkouts option fails with NPE

Providing the checkouts option on the command line currently throws an exception:

```
boot --dependencies clojure.java-time:0.3.0 --checkouts clojure.java-time:0.3.0 repl
```

```
java.lang.NullPointerException
at boot.main$dep_ns_decls.invoke(main.clj:52)
at boot.main$export_task_namespaces$fn__1667.invoke(main.clj:65)
at clojure.core$map$fn__4785.invoke(core.clj:2644)
at clojure.lang.LazySeq.sval(LazySeq.java:40)
at clojure.lang.LazySeq.seq(LazySeq.java:49)
at clojure.lang.RT.seq(RT.java:521)
at clojure.core$seq__4357.invokeStatic(core.clj:137)
at clojure.core$apply.invokeStatic(core.clj:641)
at clojure.core$mapcat.invokeStatic(core.clj:2674)
at clojure.core$mapcat.doInvoke(core.clj:2674)
at clojure.lang.RestFn.invoke(RestFn.java:423)
at boot.main$export_task_namespaces.invoke(main.clj:67)
at boot.main$_main.invoke(main.clj:179)
at clojure.lang.Var.invoke(Var.java:394)
at org.projectodd.shimdandy.impl.ClojureRuntimeShimImpl.invoke(ClojureRuntimeShimImpl.java:159)
at org.projectodd.shimdandy.impl.ClojureRuntimeShimImpl.invoke(ClojureRuntimeShimImpl.java:150)
at boot.App.runBoot(App.java:399)
at boot.App.main(App.java:488)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at Boot.main(Boot.java:258)
```