Convert all property values to strings

Allows using unquoted numeric values. Tricky to test because only
breaks when trying to run the whole thing through cssnext in the full
chain. Will figure it out later :)

src/util/defineClass.js

@@ -5,7 +5,7 @@ export default function(className, properties) {
   const decls = _.map(properties, (value, property) => {
     return postcss.decl({
       prop: _.kebabCase(property),
-      value: value,
+      value: `${value}`,
     })
   })