Don't install development gems.

For whatever reason, the 'development' group is installed by default via
bundler. This was not intentiona.

The effect is that we might be unintentionally violating a dependency's
license (term-ansicolor is GPL2) by including it in our release.
Definitely do not wish such things.

With this patch, the next run of `bin/logstash deps` shows all dev
dependencies being removed:

    Removing coveralls (0.7.0)
    Removing docile (1.1.3)
    Removing kramdown (1.3.3)
    Removing rest-client (1.6.7)
    Removing simplecov (0.8.2)
    Removing simplecov-html (0.8.0)
    Removing term-ansicolor (1.3.0)
    Removing thor (0.18.1)
    Removing tins (1.0.0)

Fixes #1372