fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-INFLIGHT-6095116
- https://snyk.io/vuln/SNYK-JS-MINIMIST-2429795
- https://snyk.io/vuln/SNYK-JS-MINIMIST-559764
- https://snyk.io/vuln/SNYK-JS-UGLIFYJS-1727251
- https://snyk.io/vuln/npm:uglify-js:20151024

package.json

@@ -48,7 +48,7 @@
     "koa-limit": "^1.0.2",
     "koa-markdown": "^2.0.2",
     "koa-maxrequests": "^1.0.0",
-    "koa-middlewares": "^2.1.0",
+    "koa-middlewares": "^6.0.0",
     "koa-mock": "^1.6.2",
     "koa-proxy": "^0.9.0",
     "koa-rewrite": "^1.1.2",
@@ -64,7 +64,7 @@
     "mz": "^2.6.0",
     "nodemailer": "^1.3.0",
     "normalize-registry-metadata": "^1.1.2",
-    "rimraf": "^2.6.2",
+    "rimraf": "^4.0.0",
     "semver": "^5.4.1",
     "sequelize": "^3.23.4",
     "ssri": "^8.0.1",