Allowing for a few generic language aliases

Related to travis-ci/travis-ci#2856
Regenerating [`./SPEC.md`](./SPEC.md)
JRuby needs to be at least 1.7.16
remove jruby for now
Merge remote-tracking branch 'origin/master' into db-feature-allow-bash-language
Merge pull request #33 from travis-ci/db-feature-allow-bash-language

Allowing for a few generic language aliases
do not rely on visit_unexpected raising an exception
add spec
Merge pull request #37 from travis-ci/rkh-no-raise-on-unexpected

do not rely on visit_unexpected raising an exception
Adding bits for the ssh known hosts addon
Merge pull request #38 from travis-ci/db-feature-ssh-known-hosts-addon

Adding bits for the ssh known hosts addon
Add 'dist' and 'group' as top-level scalar key

These are not FixedValue keys, since the values are simply
passed through to travis-worker.
It will then use the given values to select an appropriate
build image.

We can consider adding a warning when these are given, since
this key is not active for all users yet.
Add warnings for dist and group