chore(package): update rollup-plugin-postcss to version 2.3.0

Closes #68

Author: greenkeeper[bot]

package.json

@@ -126,7 +126,7 @@
     "rollup-plugin-commonjs": "^9.0.0",
     "rollup-plugin-json": "^3.0.0",
     "rollup-plugin-node-resolve": "^3.0.0",
-    "rollup-plugin-postcss": "^1.6.1",
+    "rollup-plugin-postcss": "^2.3.0",
     "rollup-plugin-scss": "^0.4.0",
     "rollup-plugin-sourcemaps": "^0.4.2",
     "rollup-plugin-typescript2": "^0.14.0",