Update proxy-agent to v4 to resolve vulnerability (#355)

closes #356 #357

Author: chadxz

.gitignore

@@ -4,3 +4,4 @@ node_modules/
 coverage/
 test/.tmp*
 examples/request_over_proxy_tmp.js
+package-lock.json

package.json

@@ -42,7 +42,7 @@
     "humanize-ms": "^1.2.0",
     "iconv-lite": "^0.4.15",
     "ip": "^1.1.5",
-    "proxy-agent": "^3.1.0",
+    "proxy-agent": "^4.0.1",
     "pump": "^3.0.0",
     "qs": "^6.4.0",
     "statuses": "^1.3.1",