Fix ns warning: remove is= use

Noticed the following warning when running cljs tests:

  WARNING: No such namespace:  at line 1 target/generated/test/cljs/schema/core_test.cljs

Pretty unclear, but compiling with latest clojurescript gives gives a better
warning:

  WARNING: Referred macro cemerick.cljs.test/is= does not exist at line 1 target/generated/test/cljs/schema/core_test.cljs
Merge pull request #75 from loganlinn/fix-namespace-warning

Fix ns warning: remove is= use