Add binstubs

[lwe]

1. Change gem bindir to exe: spec.bindir = 'exe'
2. Add binstubs for rake + rubocop
3. Invoke binstubs in .travis.yml et all

[oliverklee]

What would be the benefit of binstubs (for a Gem)?

[lwe]

Not having to shell out explicitly to bundle exec all the time when running commands, similar like you'd have in a Rails app, i.e.:

bundle exec rake test
bundle exec rubocop ...

# vs

./bin/rake test
./bin/rubocop

It's simply that my muscle memory started to favour calling things out of ./bin/... rather than using bundle exec (or a be alias) 😄