Changed 'bundle exec irb' to 'bundle console'

It pre-load gems in the following case.

Gemfile
```ruby
source "https://rubygems.org"

gem "i18n"
gem "active_support", :require => 'active_support/core_ext'
```

Inf-Ruby:run
```
irb(main):001:0> 1.day
=> 1 day
```