Make piggieback a no-op when ClojureScript isn't present

[SevereOverfl0w]

Currently, running piggieback without ClojureScript loaded gives some error about tools.reader (I think it's requiring a namespace that doesn't exist or something).

One option for working around this is to do a conditional wrap around the require, and create a no-op middleware otherwise.

Exact error:

Exception in thread "main" java.lang.ExceptionInInitializerError
        at clojure.main.<clinit>(main.java:20)
Caused by: java.io.FileNotFoundException: Could not locate clojure/tools/reader__init.class or clojure/tools/reader.clj on classpath: 
        at clojure.lang.RT.load(RT.java:432)
        at clojure.lang.RT.load(RT.java:400)
        at clojure.core$load$fn__4890.invoke(core.clj:5415)
        at clojure.core$load.doInvoke(core.clj:5414)
        at clojure.lang.RestFn.invoke(RestFn.java:408)
        at clojure.core$load_one.invoke(core.clj:5227)
        at clojure.core$load_lib.doInvoke(core.clj:5264)
        at clojure.lang.RestFn.applyTo(RestFn.java:142)
        at clojure.core$apply.invoke(core.clj:603)
        at clojure.core$load_libs.doInvoke(core.clj:5298)
        at clojure.lang.RestFn.applyTo(RestFn.java:137)
        at clojure.core$apply.invoke(core.clj:603)
        at clojure.core$require.doInvoke(core.clj:5381)
        at clojure.lang.RestFn.invoke(RestFn.java:1789)
        at cider.piggieback$eval9$loading__4784__auto____10.invoke(piggieback.clj:1)
        at cider.piggieback$eval9.invoke(piggieback.clj:1)
        at clojure.lang.Compiler.eval(Compiler.java:6511)
        at clojure.lang.Compiler.eval(Compiler.java:6501)
        at clojure.lang.Compiler.load(Compiler.java:6952)
        at clojure.lang.RT.loadResourceScript(RT.java:359)
        at clojure.lang.RT.loadResourceScript(RT.java:350)
        at clojure.lang.RT.load(RT.java:429)
        at clojure.lang.RT.load(RT.java:400)
        at clojure.core$load$fn__4890.invoke(core.clj:5415)
        at clojure.core$load.doInvoke(core.clj:5414)
        at clojure.lang.RestFn.invoke(RestFn.java:408)
        at clojure.core$load_one.invoke(core.clj:5227)
        at clojure.core$load_lib.doInvoke(core.clj:5264)
        at clojure.lang.RestFn.applyTo(RestFn.java:142)
        at clojure.core$apply.invoke(core.clj:603)
        at clojure.core$load_libs.doInvoke(core.clj:5298)
        at clojure.lang.RestFn.applyTo(RestFn.java:137)
        at clojure.core$apply.invoke(core.clj:603)
        at clojure.core$require.doInvoke(core.clj:5381)
        at clojure.lang.RestFn.invoke(RestFn.java:457)
        at user$eval3$loading__4784__auto____4.invoke(user.clj:1)
        at user$eval3.invoke(user.clj:1)
        at clojure.lang.Compiler.eval(Compiler.java:6511)
        at clojure.lang.Compiler.eval(Compiler.java:6501)
        at clojure.lang.Compiler.load(Compiler.java:6952)
        at clojure.lang.RT.loadResourceScript(RT.java:359)
        at clojure.lang.RT.loadResourceScript(RT.java:346)
        at clojure.lang.RT.maybeLoadResourceScript(RT.java:342)
        at clojure.lang.RT.doInit(RT.java:450)
        at clojure.lang.RT.<clinit>(RT.java:318)
        ... 1 more

(Reproduce by doing lein with-profile nrepl-0.6 repl)

[SevereOverfl0w]

Going to use the snapshot as my driver. Already gave it a quick spin and I'm happy with it.

It's nice not to remove piggieback whenever I connect to a non-cljs project :)