Add Travis CI before_script for install Redis 2.8.9

[Travis CI Environment](http://docs.travis-ci.com/user/ci-environment/), Redis version is 2.8.2 or 2.6.x.
[ZLEXCOUNT](http://redis.io/commands/zlexcount) command is available since 2.8.9.
So, SortedSetOperationsSpec has been failed after [this change](https://github.com/debasishg/scala-redis-nb/commit/a67e16d0dc3a70fd937204055b704e7623c5d726#diff-c21bfe19b73d4a08b76ca78e98c2fd8dR237).

    The future returned an exception of type: com.redis.protocol.package$RedisError, with message: ERR unknown command 'ZLEXCOUNT'. (SortedSetOperationsSpec.scala:240)

Now, we have to install Redis 2.8.9 before build at Travis CI.

I have confirmed the build success at my repository.
https://travis-ci.org/zaneli/scala-redis-nb/builds/24392720
[![Build Status](https://travis-ci.org/zaneli/scala-redis-nb.png?branch=travis-ci)](https://travis-ci.org/zaneli/scala-redis-nb)