Update peer dependencies

[piuccio]

When I install this package I get the warning

npm WARN firebase-functions@0.5.7 requires a peer of firebase-admin@~4.2.1 but ...

...but firebase-admin is already at version 5.0.0

[laurenzlong]

Hey @piuccio it's actually a strict dependency requirement (that's why there's ~ instead of >), so firebase-admin must be 4.2.x, nothing higher, nothing lower. Make sure you run
npm install --save firebase-admin@4.2.1
in your functions directory.

[piuccio]

I guess my point was that it's probably time for this package to update to firebase-admin 5. Any plans for that update?

[laurenzlong]

Gotcha, thanks for the clarification. Will add this to my to-do list. The reason why we don't update our dependencies automatically every time there's a new version of firebase-admin is because there can be breaking changes so we want to test extensively before upgrading.

[laurenzlong]

Submitted PR for this: #66