Revert "update async package (removes old lodash dependency)"

This reverts commit 587b777.

Author: eli-darkly

package-lock.json

@@ -25,7 +25,7 @@
  "homepage": "https://github.com/launchdarkly/node-server-sdk",
  "dependencies": {
  "@types/redis": "2.8.6",
- "async": "3.1.0",
+ "async": "2.6.0",
  "lrucache": "^1.0.3",
  "node-cache": "^3.2.1",
  "node-sha1": "0.0.1",