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
Merge pull request #1375 from jordansissel/fix/github-1372

Don't install development gems.
Disable coverage for now

This is to make tests run. Currently we are not using coverage reports,
and a recent fix to remove development dependencies from the normal
installation has caused coverage dependencies to be removed and thus
fail.

We will revert this once we figure out how to make tests run with
separate bundler groups.
Merge pull request #1376 from jordansissel/fix/test-failure-due-to-coverage-missing

Disable coverage for now
Merge git://github.com/logstash/logstash