The readme mentions switch to JRuby-1.7.2

This tripped me up when I was getting started.
See commit 59dfd5159c7bfc97872ae4b3a1f471690df0fe8c
Merge pull request #410 from romanzenka/master

Trivial update to the readme file mentioning switch to JRuby-1.7.2
- Add note of redis 2.6 requirement for new batch_count input setting
- Have bin/logstash fetch jruby if ruby is not found.

- note redis 2.6 requirement for new batch_count on redis input

- randomize batch size

- only build-jruby if it's not there. This avoids the 'make: Nothing to
be done for ...' message