Rebuild Gemfile.lock

You can't install Loomio without doing `bundle update` first to build the Gemfile.lock..

```bash
~/Code/loomio ♬  bundle install
Fetching gem metadata from http://rubygems.org/......
Fetching gem metadata from http://rubygems.org/..
Could not find newrelic_rpm-3.5.6.46 in any of the sources
```

With this update, a `bundle install` should just install the gems listed in the lockfile, eliminating the need for making an extra commit to the repo on top of a release. This improves consistency and the knowledge that the commit on your server is going to be the same commit that a given version tag points to.