fix(package): update @bootstrap-styled/css-mixins to version 1.3.1

Author: greenkeeper[bot]

package.json

@@ -221,7 +221,7 @@
     "build:readme"
   ],
   "dependencies": {
-    "@bootstrap-styled/css-mixins": "1.3.0",
+    "@bootstrap-styled/css-mixins": "1.3.1",
     "@bootstrap-styled/utils": "^1.7.0"
   },
   "publishConfig": {