Update agent-base to v4.3 (#69)

See TooTallNate/node-agent-base#25 and TooTallNate/node-agent-base#27.

Author: BYK

package.json

@@ -23,7 +23,7 @@
     "url": "https://github.com/TooTallNate/node-https-proxy-agent/issues"
   },
   "dependencies": {
-    "agent-base": "^4.2.0",
+    "agent-base": "^4.3.0",
     "debug": "^3.1.0"
   },
   "devDependencies": {