Update .travis.yml

[junaruga]

I updated rubies to latest version.

I added ruby-head and jruby-head as allow_failures in .travis.yml. I think this is useful. Because we can prepare before next version Ruby 2.5 release.

We can see this kind of logic in rails, rspec and cucumber and etc. https://github.com/rails/rails/blob/master/.travis.yml https://github.com/rspec/rspec-core/blob/master/.travis.yml https://github.com/cucumber/cucumber-ruby/blob/master/.travis.yml

Is it possible to add it?

Thanks.

[junaruga]

https://travis-ci.org/banister/method_source/builds/223576399 Error at Ruby 1.9.3, rbx-19mode, rbx-18mode,

Ruby 1.9.3

NoMethodError: undefined method `spec' for nil:NilClass
An error occurred while installing method_source (0.8.1), and Bundler cannot
continue.
Make sure that `gem install method_source -v '0.8.1'` succeeds before bundling.

I added bundler_args: "--verbose" to see the detail during bundle install.

rbx-19mode, rbx-18mode

Requested binary installation but no rubies are available to download, consider skipping --binary flag.

https://docs.travis-ci.com/user/languages/ruby/

Note that the syntax of rbx-19mode is not supported anymore.

However I do not know any other available rbx format. Because recently I faced the same situation for rbx-2, rbx-3, and rbx-3.74 for another rubygem package.

I referred reliable project rspec-core's way for rbx. https://github.com/rspec/rspec-core/blob/master/.travis.yml

[junaruga]

Ruby 1.9.3 https://travis-ci.org/banister/method_source/builds/225204983

The reason of the error looks this. http://stackoverflow.com/questions/33733476/nomethoderror-undefined-method-spec-for-nilnilclass-active-utils-factory

[junaruga]

Hello @banister all the tests are passed. Could you merge this PR? Thank you.

[banister]

@junaruga thanks!