chore(package): update autoprefixer to version 7.1.5

Author: greenkeeper[bot]

package.json

@@ -17,7 +17,7 @@
     "url": "https://github.com/textpattern/textpattern-jquery-ui-theme/issues"
   },
   "devDependencies": {
-    "autoprefixer": "7.1.4",
+    "autoprefixer": "7.1.5",
     "grunt": "~1.0",
     "grunt-cli": "~1",
     "grunt-concurrent": "2.3.1",