fix: package.json & .snyk to reduce vulnerabilities (#11)

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

.snyk

@@ -488,3 +488,8 @@ patch:
         patched: '2019-07-04T05:34:14.882Z'
     - babel-preset-stage-0 > babel-preset-stage-1 > babel-preset-stage-2 > babel-preset-stage-3 > babel-plugin-transform-async-to-generator > babel-helper-remap-async-to-generator > babel-helper-function-name > babel-template > babel-traverse > babel-types > lodash:
         patched: '2019-07-04T05:34:14.882Z'
+  SNYK-JS-HTTPSPROXYAGENT-469131:
+    - snyk > proxy-agent > https-proxy-agent:
+        patched: '2019-10-05T05:33:44.811Z'
+    - snyk > proxy-agent > pac-proxy-agent > https-proxy-agent:
+        patched: '2019-10-05T05:33:44.811Z'

package.json

@@ -19,7 +19,7 @@
     "react-addons-test-utils": "^0.14.7",
     "react-dom": "^16.0.1",
     "react-router": "^2.0.0",
-    "snyk": "^1.189.0"
+    "snyk": "^1.231.0"
   },
   "devDependencies": {
     "babel-core": "^6.5.1",