fix: pin proxy-agent & get-uri deps to not-break node 4

as `proxy-agent` -> `pac-proxy-agent` -> `get-uri@^2.0.0`
and in `get-uri@2.0.3`, the `debug` dep was bumped from 2 to 4
(TooTallNate/node-get-uri@126d4f5)
and this doesn't support node 4 (throwing `SyntaxError`)

note this commit adds `get-uri` as a direct dep, although it isn't

package.json

@@ -56,13 +56,14 @@
     "chalk": "^2.4.1",
     "configstore": "^3.1.2",
     "debug": "^3.1.0",
+    "get-uri": "2.0.2",
     "hasbin": "^1.2.3",
     "inquirer": "^3.0.0",
     "lodash": "^4.17.5",
     "needle": "^2.2.4",
     "opn": "^5.2.0",
     "os-name": "^2.0.1",
-    "proxy-agent": "^2.0.0",
+    "proxy-agent": "2.3.1",
     "proxy-from-env": "^1.0.0",
     "recursive-readdir": "^2.2.2",
     "semver": "^5.5.0",