Change firebase-admin dependency to be less strict (#21)

Author: laurenzlong

package.json

@@ -46,7 +46,7 @@
  },
  "peerDependencies": {
  "firebase-functions": ">1.0.1",
- "firebase-admin": "^5.12.0"
+ "firebase-admin": ">5.12.0"
  },
  "engines": {
  "node": ">=6.0.0"