[Plugin Generator] Update template files

[ycombinator]

I recently (about a couple weeks ago) created a new logstash filter plugin using the plugin generator (bin/logstash-plugin generate ...). In the PR review for my plugin I got some feedback on elements that were generated by the plugin generator. So I figured I should address that feedback in the plugin generator templates as well, in order to aid future plugin authors who use the plugin generator. This PR does that.

Specifically, this PR:

• Updates the instructions about how we build docs to no longer say that we parse comments from plugin code.
• Adds a snippet to the Gemfile that allows testing against multiple branch versions.
• Adds the config file for Travis CI.

[dedemorton]

Doc changes LGTM. However, I'm wondering if the plugin generator also needs to be modified to create the index.asciidoc skeleton files for each plugin type. I've created PRs to update the example repos, so you could take the files from there. TBH, I haven't used the generator for a long time, so I'm not even sure what it spits out (or should spit out) these days. Here are the PRs:

logstash-plugins/logstash-input-example#12
logstash-plugins/logstash-codec-example#9
logstash-plugins/logstash-output-example#7
logstash-plugins/logstash-filter-example#9

[ycombinator]

Sure, happy to throw skeleton index.asciidoc files for each plugin as part of this PR. I assume I can just copy over the ones from the links you pasted?

[jakelandis]

For the template, I would suggest to leave off 6.0. I need to update (add or replace) to 6.2 across the board. for the template, I think master, 6.x, and 5.6 have a decent longevity.

[jakelandis]

One minor (optional) suggestion around the travis template, and +1 for the skeleton index.asciidoc.

LGTM , and thanks for taking care of this !

[andrewvc]

Jenkins, please test this