Adding bundle install as a step for installation

Redmine installation plugin tutorial doesn't say anything about those
gem dependencies, so I think the plugin should tell the users to do
that
I had sugested just because we can already have a redmine instance deployed, and this plugin requires a gem that normally redmine doesn't.

Not sure how would be the best way to say it, but doing a `bundle install` when the environment has all the gems already installed do no harm.

Fell free not to merge it, it was just a sugestion =)