Log commit hash on slasher startup

[0xKiwi]

What type of PR is this?
Improvement

What does this PR do? Why is it needed?
This PR adds a log on startup displaying the slasher client version and removes a duplicate insecure gRPC log.

[2020-06-04 18:06:24]  INFO node: Checking DB database-path=/home/kiwi/.eth2
[2020-06-04 18:06:24]  INFO node: Starting slasher client version=Prysm/Git commit: 0171f679c868dbaa136d111dbf57490a8ce4279f. Built at: 2020-06-04T18:06:24-04:00
[2020-06-04 18:06:24]  WARN beaconclient: You are using an insecure gRPC connection to beacon chain! Please provide a certificate and key to use a secure connection
[2020-06-04 18:06:24]  INFO beaconclient: Successfully started gRPC connection
[2020-06-04 18:06:24]  INFO RPC-API listening on port address=0.0.0.0:4002

Which issues(s) does this PR fix?
Fixes #6082