Fix missing logger app in KV Server

The end of chapter 9 suggests suppressing the logger during the KV
Server restart in the test setup, but :logger was not included as
an OTP application dependency in the mix.exs file, resulting in an
error.

This commit adds the :logger dependency back to the kv_server
applications.
Merge pull request #378 from readysetmark/master

Fixes #370 missing logger app in KV Server