Liquid 3.0

This PR isn't really intended to get merged, I just want to show @parkr what is necessary to make the Jekyll tests pass with current Liquid master (to be released as 3.0 soon).

```
/usr/lib/shopify-ruby/2.1.2-shopify1/bin/ruby -I"lib:lib:test" -I"/home/vagrant/.gem/ruby/2.1.2/gems/rake-10.3.2/lib" "/home/vagrant/.gem/ruby/2.1.2/gems/rake-10.3.2/lib/rake/rake_test_loader.rb" "test/**/test_*.rb"
method adapters is deprecated. use profiles instead
Notice: for 10x faster LSI support, please install http://rb-gsl.rubyforge.org/
Run options:

# Running tests:

Finished tests in 42.846838s, 9.7323 tests/s, 16.4773 assertions/s.
417 tests, 706 assertions, 0 failures, 0 errors, 0 skips

ruby -v: ruby 2.1.2p95-shopify (development) [x86_64-linux]
Coverage report generated for Unit Tests to /home/vagrant/src/jekyll/coverage. 1653 / 1925 LOC (85.87%) covered.
/usr/lib/shopify-ruby/2.1.2-shopify1/bin/ruby -S bundle exec cucumber  --profile travis
Using the travis profile...
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149 scenarios (149 passed)
1095 steps (1095 passed)
2m32.512s
```

Also see https://github.com/Shopify/liquid/pull/390. The only problem is https://github.com/Shopify/liquid/pull/321, which refactored the Liquid::Block constructor in a not-100%-backwards compatible way.

cc @dylanahsmith
@parkr, we just released 3.0.0.rc1 to Rubygems. We will test it in Shopify for a week or two and if there are no regressions, release 3.0.0. If you could do some tests on big Jekyll sites with this branch and let us know if you find anything, that would be awesome. / cc @arthurnn