when doing `lein run` avoid that ClassNotFoundException: ships.core

without this, only lein uberjar (and maybe also lein jar) would compile
ships.core into a .class; lein compile or lein javac or lein run
wouldn't

tested with lein 2.2.0

without this:
```
s:\workspace.eclipse.2k13_2\fightingsail>lein run
Exception in thread "main" java.lang.RuntimeException: java.lang.ClassNotFoundException: ships.core, compiling:(ships/core.clj:38)
        at clojure.lang.Compiler.analyzeSeq(Compiler.java:6416)
        at clojure.lang.Compiler.analyze(Compiler.java:6216)
        at clojure.lang.Compiler.access$100(Compiler.java:37)
        at clojure.lang.Compiler$LetExpr$Parser.parse(Compiler.java:5837)
        at clojure.lang.Compiler.analyzeSeq(Compiler.java:6409)
        at clojure.lang.Compiler.analyze(Compiler.java:6216)
        at clojure.lang.Compiler.analyzeSeq(Compiler.java:6397)
        at clojure.lang.Compiler.analyze(Compiler.java:6216)
        at clojure.lang.Compiler.analyzeSeq(Compiler.java:6397)
        at clojure.lang.Compiler.analyze(Compiler.java:6216)
        at clojure.lang.Compiler.analyze(Compiler.java:6177)
        at clojure.lang.Compiler$BodyExpr$Parser.parse(Compiler.java:5572)
        at clojure.lang.Compiler$FnMethod.parse(Compiler.java:5008)
        at clojure.lang.Compiler$FnExpr.parse(Compiler.java:3629)
        at clojure.lang.Compiler.analyzeSeq(Compiler.java:6407)
        at clojure.lang.Compiler.analyze(Compiler.java:6216)
        at clojure.lang.Compiler.analyzeSeq(Compiler.java:6397)
        at clojure.lang.Compiler.analyze(Compiler.java:6216)
        at clojure.lang.Compiler.access$100(Compiler.java:37)
        at clojure.lang.Compiler$DefExpr$Parser.parse(Compiler.java:492)
        at clojure.lang.Compiler.analyzeSeq(Compiler.java:6409)
        at clojure.lang.Compiler.analyze(Compiler.java:6216)
        at clojure.lang.Compiler.analyze(Compiler.java:6177)
        at clojure.lang.Compiler.eval(Compiler.java:6469)
        at clojure.lang.Compiler.load(Compiler.java:6902)
        at clojure.lang.RT.loadResourceScript(RT.java:357)
        at clojure.lang.RT.loadResourceScript(RT.java:348)
        at clojure.lang.RT.load(RT.java:427)
        at clojure.lang.RT.load(RT.java:398)
        at clojure.core$load$fn__4610.invoke(core.clj:5386)
        at clojure.core$load.doInvoke(core.clj:5385)
        at clojure.lang.RestFn.invoke(RestFn.java:408)
        at clojure.core$load_one.invoke(core.clj:5200)
        at clojure.core$load_lib.doInvoke(core.clj:5237)
        at clojure.lang.RestFn.applyTo(RestFn.java:142)
        at clojure.core$apply.invoke(core.clj:602)
        at clojure.core$load_libs.doInvoke(core.clj:5271)
        at clojure.lang.RestFn.applyTo(RestFn.java:137)
        at clojure.core$apply.invoke(core.clj:602)
        at clojure.core$require.doInvoke(core.clj:5352)
        at clojure.lang.RestFn.invoke(RestFn.java:408)
        at user$eval3.invoke(NO_SOURCE_FILE:1)
        at clojure.lang.Compiler.eval(Compiler.java:6465)
        at clojure.lang.Compiler.eval(Compiler.java:6454)
        at clojure.lang.Compiler.eval(Compiler.java:6431)
        at clojure.core$eval.invoke(core.clj:2795)
        at clojure.main$eval_opt.invoke(main.clj:296)
        at clojure.main$initialize.invoke(main.clj:315)
        at clojure.main$null_opt.invoke(main.clj:348)
        at clojure.main$main.doInvoke(main.clj:426)
        at clojure.lang.RestFn.invoke(RestFn.java:421)
        at clojure.lang.Var.invoke(Var.java:405)
        at clojure.lang.AFn.applyToHelper(AFn.java:163)
        at clojure.lang.Var.applyTo(Var.java:518)
        at clojure.main.main(main.java:37)
Caused by: java.lang.RuntimeException: java.lang.ClassNotFoundException: ships.core
        at clojure.lang.Util.runtimeException(Util.java:165)
        at clojure.lang.RT.classForName(RT.java:2017)
        at clojure.lang.Compiler$HostExpr.maybeClass(Compiler.java:929)
        at clojure.lang.Compiler$HostExpr.access$400(Compiler.java:710)
        at clojure.lang.Compiler$NewExpr$Parser.parse(Compiler.java:2428)
        at clojure.lang.Compiler.analyzeSeq(Compiler.java:6409)
        ... 54 more
Caused by: java.lang.ClassNotFoundException: ships.core
        at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
        at clojure.lang.DynamicClassLoader.findClass(DynamicClassLoader.java:61)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:266)
        at clojure.lang.RT.classForName(RT.java:2013)
        ... 58 more
```

with this fix, both `lein run` and `lein compile` will make ships.core class (at least) and `lein run` will run without errors.