Allow GHC-8.8.4

Currently, GHC-8.8.4 is not allowed:

```
➜ stack build

Error: While constructing the build plan, the following exceptions were encountered:

In the dependencies for fcm-client-0.2.0.0:
    base-4.13.0.0 from stack configuration does not match >=4.10.1.0 && <4.13  (latest matching version
                  is 4.12.0.0)
needed since fcm-client is a build target.
```