Don't require Bundler to use binstubs

[Spring](https://github.com/jonleighton/spring#usage) installs binstubs as `bin/cucumber` with this content:

```bash
#!/usr/bin/env bash
exec $(dirname $0)/spring cucumber "$@"
```

To execute this, I would need to start guard-cucumber with this command:

```
guard 'cucumber', binstubs: true, bundler: false
```

However, this was not possible before, because `binstubs: true` implied `bundler: true`. I'm sure there is a good reason for this, and feel free to reject this PR, this is just something that works for me, and I expect helps others as well.