Remove Redis version from test instructions

The version of Redis we test against is already listed at the top of the
README.

Change-Id: Ib6f3dcc2a756cd6581e6dffe3228e649233986d5
Reviewed-on: http://gerrit.causes.com/45136
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Tested-by: Shane da Silva <shane.dasilva@brigade.com>