Canonicalize travis ruby versions
Do not intall stackprof under jruby nor rubinius
rbx-19mode is not supported anymore
Merge pull request #374 from Shopify/remove-duplicate-ruby-version

Give some :heart: to travis.yml
Allow tag classes to be reloaded when using Liquid.cache_classes is false

Because Liquid keeps a reference to tag classes, Rails class reloading may
cause problems with custom tags. This commit introduces a setting that
allows these classes to be resolved when required.