Fix mocha compilers deprecation warning

Author: marvinhagemeister

package.json

@@ -12,7 +12,7 @@
     "transpile": "microbundle src/index.js -f es,umd --target web --external preact",
     "transpile:jsx": "microbundle src/jsx.js -o dist/jsx.js --target web --external none && microbundle dist/jsx.js -o dist/jsx.js -f cjs",
     "copy-typescript-definition": "copyfiles -f src/index.d.ts dist",
-    "test": "eslint src test && tsc && mocha --compilers js:babel-register test/**/*.js",
+    "test": "eslint src test && tsc && mocha -r babel-core/register test/**/*.js",
     "prepublish": "npm run build",
     "release": "npm run build && git commit -am $npm_package_version && git tag $npm_package_version && git push && git push --tags && npm publish"
   },