Add Bundler.require(:development) for default rake tasks

[andrehjr]

[Minor issue]

No more need to execute bundle exec as bundler will be already
loaded it.

Also fix the following error if someone tried to call rake spec directly, because code would exec that you'd call 'bundle exec'.

➜  tugboat git:(master) ✗ rake spec
rake aborted!
LoadError: cannot load such file -- cucumber/rake/task
/Users/andrehjr/work/tugboat/Rakefile:6:in `<top (required)>'

[petems]

@andrehjr Thank you for your contribution! 👍