fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-AXIOS-174505

package.json

@@ -11,7 +11,7 @@
     "docs": "documentation build src/api.js -f html -o docs/api && documentation build src/api.js -f md -o docs/api.md"
   },
   "dependencies": {
-    "axios": "^0.15.3",
+    "axios": "^0.18.1",
     "babel-plugin-transform-object-rest-spread": "^6.26.0",
     "connect-history-api-fallback": "^1.3.0",
     "express": "^4.16.1",