prefix bitters install with bundle exec

I ran into the issue described here: https://github.com/centresource/generator-playbook/issues/19

On my machine the 'bitters install' command failed during generation. I think the root cause is the fact that my bundler config uses a local BUNDLE_PATH.

```shell
~ cat ~/.bundle/config
---
BUNDLE_PATH: .bundle
BUNDLE_BIN: .bundle/binstubs
BUNDLE_JOBS: '7'

~ echo $PATH
... :.bundle/binstubs: ...
```

So, if I try to run bitters in a subdirectory of my project (as the generator does), the executable is not found. Prefixing the command with bundle exec fixes this.

I am very new to yeoman so if there is a better way to fix, please let me know!