Remove active_support from the development dependancies.

@arthurnn for review

## Problem

When running the test suite with active_support 4.1 the test suite is failing with

```
Warning: you should require 'minitest/autorun' instead.
Warning: or add 'gem "minitest"' before 'require "minitest/autorun"'
From:
  /Users/dylansmith/.rbenv/versions/2.1.1/lib/ruby/2.1.0/test/unit.rb:1:in `require'
  /Users/dylansmith/.rbenv/versions/2.1.1/lib/ruby/2.1.0/test/unit.rb:1:in `<top (required)>'
  /Users/dylansmith/src/liquid/test/test_helper.rb:3:in `require'
  /Users/dylansmith/src/liquid/test/test_helper.rb:3:in `<top (required)>'
  /Users/dylansmith/src/liquid/test/integration/assign_test.rb:1:in `require'
  /Users/dylansmith/src/liquid/test/integration/assign_test.rb:1:in `<top (required)>'
  /Users/dylansmith/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/rake-10.2.2/lib/rake/rake_test_loader.rb:15:in `require'
  /Users/dylansmith/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/rake-10.2.2/lib/rake/rake_test_loader.rb:15:in `block in <main>'
  /Users/dylansmith/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/rake-10.2.2/lib/rake/rake_test_loader.rb:4:in `select'
  /Users/dylansmith/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/rake-10.2.2/lib/rake/rake_test_loader.rb:4:in `<main>'
MiniTest::Unit::TestCase is now Minitest::Test. From /Users/dylansmith/.rbenv/versions/2.1.1/lib/ruby/2.1.0/test/unit/testcase.rb:8:in `<module:Unit>'
/Users/dylansmith/.rbenv/versions/2.1.1/lib/ruby/2.1.0/test/unit.rb:676:in `<class:Runner>': undefined method `_run_suite' for class `Test::Unit::Runner' (NameError)
	from /Users/dylansmith/.rbenv/versions/2.1.1/lib/ruby/2.1.0/test/unit.rb:261:in `<module:Unit>'
	from /Users/dylansmith/.rbenv/versions/2.1.1/lib/ruby/2.1.0/test/unit.rb:15:in `<module:Test>'
	from /Users/dylansmith/.rbenv/versions/2.1.1/lib/ruby/2.1.0/test/unit.rb:7:in `<top (required)>'
	from /Users/dylansmith/src/liquid/test/test_helper.rb:3:in `require'
	from /Users/dylansmith/src/liquid/test/test_helper.rb:3:in `<top (required)>'
	from /Users/dylansmith/src/liquid/test/integration/assign_test.rb:1:in `require'
	from /Users/dylansmith/src/liquid/test/integration/assign_test.rb:1:in `<top (required)>'
	from /Users/dylansmith/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/rake-10.2.2/lib/rake/rake_test_loader.rb:15:in `require'
	from /Users/dylansmith/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/rake-10.2.2/lib/rake/rake_test_loader.rb:15:in `block in <main>'
	from /Users/dylansmith/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/rake-10.2.2/lib/rake/rake_test_loader.rb:4:in `select'
	from /Users/dylansmith/.rbenv/versions/2.1.1/lib/ruby/gems/2.1.0/gems/rake-10.2.2/lib/rake/rake_test_loader.rb:4:in `<main>'
rake aborted!
```

It looks like rails 4.1 doesn't play nice with test-unit which is used by liquid.

## Solution

The only place active support was being used was in the benchmark, so I changed the benchmark to completely removed it as a development dependancy.