Update to latest release, drop the chown

The permissions are now [fixed](criteo/cassandra_exporter#68) in upstream repo, so we can drop [the chown](#4 (review)):
```
porridge@beczulka:~$ docker run criteord/cassandra_exporter:2.3.2 ls -l /opt/cassandra_exporter
Unable to find image 'criteord/cassandra_exporter:2.3.2' locally
2.3.2: Pulling from criteord/cassandra_exporter
bc51dd8edc1b: Pull complete 
d1d06863bb82: Pull complete 
adf4d10a782b: Pull complete 
7db4424ff270: Pull complete 
e008a301333c: Pull complete 
10121b885a52: Pull complete 
0d34188e8091: Pull complete 
fc3c7604e1c5: Pull complete 
019737f36353: Pull complete 
a26e5d6f647b: Pull complete 
3b6b59aecfbd: Pull complete 
Digest: sha256:9fe7cfef9b03ca294c80e5da098add4698c4103bbddf382dd93d2e01a073d6e3
Status: Downloaded newer image for criteord/cassandra_exporter:2.3.2
total 4760
-rw-rw-r-- 1 root root 4871483 Feb 10 13:31 cassandra_exporter.jar
porridge@beczulka:~$ 
```

images/Dockerfile.prometheus-exporter

@@ -1,3 +1 @@
-FROM criteord/cassandra_exporter:2.2.1
-
-RUN chown 999:999 /opt/cassandra_exporter/cassandra_exporter.jar
+FROM criteord/cassandra_exporter:2.3.2