Define a template for travis.yml

In all plugin the travis.yml file is updated adhoc and has different format, jruby version...
It would be good to have a default one that would be used by almost all plugins
I understand that yaml does not care about the order of keys but it would be nice to have it for human eyes.

My starting proposal is the following where jdk and rvm are already a list, in case we decide to add jruby9k or another jvm in the future

```

---
sudo: false
language: ruby
cache: bundler
jdk:
- oraclejdk8
rvm:
- jruby-1.7.25
before_script:
- bundle exec rake vendor
script:
- bundle exec rspec spec

```

questions:
- I see a lot of `before_install: []` is it needed ?
- Sometimes there is a `before_script: bundle exec rake vendor` should it be a default ?

@ph @guyboertje anything to add to this ?

Start by inventorying the travis.yml files across the plugins and comparing their Ruby/JRuby, JDK, before_install, before_script, and script settings with the proposed template. Resolve which settings should be shared defaults and what variation plugins require. Done means a documented template and a consistent adoption plan for almost all plugins.

Written by the indexing model from the issue text.