Tell to run bundle in the Heroku deploy instructions

Without running first `bundle` I get the following error when running `heroku config:add SECRET_TOKEN="$(bundle exec rake secret)"`

/Users/florent2/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/bundler-1.3.5/lib/bundler/spec_set.rb:92:in `block in materialize': Could not find i18n-0.6.1 in any of the sources (Bundler::GemNotFound)
Merge pull request #513 from Florent2/patch-1

Tell to run `bundle` in the Heroku deploy instructions