fix(package): update postcss to version 7.0.31

Closes #6

Author: greenkeeper[bot]

package.json

@@ -25,7 +25,7 @@
   },
   "homepage": "https://github.com/nelsonpecora/postcss-get-sass-variables#readme",
   "dependencies": {
-    "postcss": "^6.0.2"
+    "postcss": "^7.0.31"
   },
   "devDependencies": {
     "chai": "^4.0.2",