Added -t option for bundle gem to generate a .travis.yml file with the current Ruby version

[ndbroadbent]

A lot of open source gems are running their tests on Travis CI.

This PR adds the option to generate an minimal (but functional) .travis.yml config file, so you can start testing on Travis CI right away.

Let me know if you think we should make this a default file, and remove the option. Gems are usually open source, so it might be cool to encourage testing on Travis CI.

[rohitarondekar]

This sounds like a nice idea. But lets keep it as an option IMHO.

One nitpick regarding the banner for the option ― I think something like this will read better: "Generate a default configuration for Travis CI."

[ndbroadbent]

I'm also happy with keeping in an option. Great suggestion for the banner, yours is a lot better :)

[rohitarondekar]

There is already an option assigned for -t on bundler master. It's supposed to generate a test folder. Before you make changes I think it will be best to ask @indirect if this PR is a desired feature? :)

[indirect]

@ndbroadbent can you just rebase and then add the Travis file as part of the -t/--test switch? It only makes sense to me to have a travis file if your gem has tests. :)

[ndbroadbent]

Rebased, and added to the --test option. (Doesn't hurt to have even if the gem isn't tested on Travis.)

[indirect]

Sounds reasonable to me. Thanks!