Cassandra version not working

I had some little issue when i tried the quick start guide.

Edited a little bit the Dockerfile.

```
FROM mashape/docker-java7
MAINTAINER Marco Palladino, marco@mashape.com

ENV CASSANDRA_VERSION 2.2.4

# adding repo to yum
RUN echo -e "[datastax]\nname = DataStax Repo for Apache Cassandra\nbaseurl = http://rpm.datastax.com/community\nenabled = 1\ngpgcheck = 0" > /etc/yum.repos.d/datastax.repo

RUN yum -y install dsc22-$CASSANDRA_VERSION cassandra22-$CASSANDRA_VERSION

RUN sed -i -r -e 's@^rpc_address.*$@rpc_address: 0\.0\.0\.0@' /etc/cassandra/conf/cassandra.yaml
RUN sed -i -r -e 's@^# broadcast_rpc_address.*$@broadcast_rpc_address: 127\.0\.0\.1@' /etc/cassandra/conf/cassandra.yaml

VOLUME ["/var/lib/cassandra", "/etc/cassandra"]

CMD ["cassandra", "-f"]

EXPOSE 22 8888 7000 7001 7199 9042 9160
```

I send you a pull request asap

Start with the quick start guide and the Dockerfile shown in the issue. Reproduce the reported failure around Cassandra 2.2.4 and determine what is not working during the image build or startup. Done means the documented quick start works with the corrected Cassandra setup and the result is clearly documented.

Written by the indexing model from the issue text.