Fix ns form to comply with cljs.core.specs.alpha

When requiring the cljs.core.specs.alpha namespace with ClojureScript 1.10, the ns form in com.rpl.specter.navs fails spec validation and produces a compile error, because the nested :clj reader conditionals result in an empty :use clause.

Moving the reader conditional up to enclose :use fixes this.

Relevant spec: https://github.com/clojure/clojurescript/blob/b11cbeefa5c148b256bcc0942d714c23ab4c6c81/src/main/cljs/cljs/core/specs/alpha.cljc#L170

Additional info: https://clojurescript.org/news/2018-03-26-release#_core_specs_alpha

Compiler error message:

----  Could not Analyze  jar:file:/Users/gnl/.m2/repository/com/rpl/specter/1.1.0/specter-1.1.0.jar!/com/rpl/specter/navs.cljc   line:1  column:1  ----

  Call to cljs.core/ns-special-form did not conform to spec:
In: [2 0] val: :use fails spec: :cljs.core.specs.alpha/ns-refer-clojure at: [:args :clauses :refer-clojure :clause] predicate: #{:refer-clojure}
In: [2 0] val: :use fails spec: :cljs.core.specs.alpha/ns-require at: [:args :clauses :require :clause] predicate: #{:require}
In: [2 0] val: :use fails spec: :cljs.core.specs.alpha/ns-require-macros at: [:args :clauses :require-macros :clause] predicate: #{:require-macros}
In: [2 0] val: :use fails spec: :cljs.core.specs.alpha/ns-import at: [:args :clauses :import :clause] predicate: #{:import}
In: [2] val: () fails spec: :cljs.core.specs.alpha/ns-use at: [:args :clauses :use :libs] predicate: (alt :libspec :cljs.core.specs.alpha/use-libspec :flag #{:verbose :reload :reload-all}),  Insufficient input
In: [2 0] val: :use fails spec: :cljs.core.specs.alpha/ns-use-macros at: [:args :clauses :use-macros :clause] predicate: #{:use-macros}

----  Analysis Error : Please see jar:file:/Users/gnl/.m2/repository/com/rpl/specter/1.1.0/specter-1.1.0.jar!/com/rpl/specter/navs.cljc  ----

redplanetlabs / specter

Fix ns form to comply with cljs.core.specs.alpha #253