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
Being more explicit on `bundle install` step

Explaining better why a `bundle install` might be necessary.

Because this plugin requires `aws-sdk`, a redmine instalation that is already running should install it.

If all dependencies are already installed a new `bundle install` would install anything else.
Merge pull request #26 from marlonandrade/master

Adding bundle install as a step for installation