Add prettier as a dev dependency (#135)

Previously, the project depended on prettier, but it was not listed
as a dev dependency, so the project was expecting it to be
installed globally, which is not a reasonable expectation. This
commit adds the package as a dev dependency so that the git hooks
defined in husky can run properly without a global dependency.

package.json

@@ -32,6 +32,7 @@
     "expect.js": "0.3.1",
     "husky": "0.14.3",
     "lint-staged": "5.0.0",
-    "mocha": "4.0.1"
+    "mocha": "4.0.1",
+    "prettier": "2.0.5"
   }
 }