Explicitly default :npm-deps to true

[mfikes]

With the next release of ClojureScript, the default of :npm-deps will change from true to false.

A consequence is that Reagent-based apps will exhibit an issue if users upgrade to this (future) ClojureScript version (this is with lein figwheel ios):

----  Could not Analyze  target/ios/reagent/impl/component.cljs  ----

  No such namespace: create-react-class, could not locate create_react_class.cljs, create_react_class.cljc, or JavaScript source providing "create-react-class" (Please check that namespaces with dashes use underscores in the ClojureScript file name)

The fix is to simply explicitly specify the :npm-deps value to true.

We many not want to do this until bhauman/lein-figwheel#715