chore(package): update semver to version 6.1.0

Closes #67

Author: greenkeeper[bot]

Diff for: package.json

@@ -82,7 +82,7 @@
     "postcss-loader": "^2.0.8",
     "postcss-url": "^7.2.1",
     "rimraf": "^2.6.0",
-    "semver": "^5.3.0",
+    "semver": "^6.1.0",
     "shelljs": "^0.7.6",
     "uglifyjs-webpack-plugin": "^1.3.0",
     "url-loader": "^0.5.8",