Adds bundler `bump` completion

Adds `bundle update --source ...` completion.

```bash
$ bump |
activesupport     database_cleaner  mongoid           rack-test         shotgun           sinatra           therubyracer
bson_ext          httparty          pry-byebug        rspec             simplecov         sinatra-contrib

bump r |
rack-test  rspec

bump rspec
Fetching gem metadata from https://rubygems.org/.......
Fetching additional metadata from https://rubygems.org/..
...
```