Fix let-syntax bug: java.util.NoSuchElementException: head of empty list

    lisp>>(let [x 1] x)
    Could not parse - `(' expected but `x' found
    java.util.NoSuchElementException: head of empty list
            at scala.collection.immutable.Nil$.head(List.scala:337)
            at scala.collection.immutable.Nil$.head(List.scala:334)
            at net.jaratec.spl.Repl$.repl2(Repl.scala:87)
            at net.jaratec.spl.Repl$.main(Repl.scala:19)
            at net.jaratec.spl.Repl.main(Repl.scala)
            at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
            at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
            at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
            at java.lang.reflect.Method.invoke(Method.java:606)
            at sbt.Run.invokeMain(Run.scala:72)
            at sbt.Run.run0(Run.scala:65)
            at sbt.Run.sbt$Run$$execute$1(Run.scala:54)
            at sbt.Run$$anonfun$run$1.apply$mcV$sp(Run.scala:58)
            at sbt.TrapExit$.sbt$TrapExit$$executeMain$1(TrapExit.scala:33)
            at sbt.TrapExit$$anon$1.run(TrapExit.scala:42)