fix: package.json & .snyk to reduce vulnerabilities

The following vulnerabilities are fixed with a Snyk patch:
- https://snyk.io/vuln/SNYK-JS-HTTPSPROXYAGENT-469131

Author: snyk-bot

.snyk

@@ -12,3 +12,8 @@ patch:
         patched: '2019-07-04T00:17:06.178Z'
     - styled-components > babel-plugin-styled-components > @babel/helper-annotate-as-pure > @babel/types > lodash:
         patched: '2019-07-04T00:17:06.178Z'
+  SNYK-JS-HTTPSPROXYAGENT-469131:
+    - snyk > proxy-agent > https-proxy-agent:
+        patched: '2019-10-05T00:17:05.679Z'
+    - snyk > proxy-agent > pac-proxy-agent > https-proxy-agent:
+        patched: '2019-10-05T00:17:05.679Z'

package.json

@@ -53,7 +53,7 @@
   "dependencies": {
     "classnames": "^2.2.6",
     "prop-types": "^15.7.2",
-    "snyk": "^1.222.0",
+    "snyk": "^1.231.0",
     "styled-components": "^4.3.2",
     "styled-system": "^5.1.1"
   },