This allows you to work on the gem while maintaining compatibility with all supported versions of AR.

The good news is there is only one spec failure with AR 4.1.1 right now:

```
# Running:

.......................S...................E........

Finished in 0.253729s, 204.9431 runs/s, 437.4746 assertions/s.

  1) Error:
Common Table Expression queries::.from_cte(common_table_expression_hash)#test_0001_generates an expression with the CTE as the main table:
NoMethodError: undefined method `attribute_alias?' for #<CTEProxy:0x0000010235dfb0>
    /Users/pboling/.rvm/gems/ruby-2.1.2@foss/gems/activerecord-4.1.1/lib/active_record/relation/predicate_builder.rb:11:in `block in resolve_column_aliases'
    /Users/pboling/.rvm/gems/ruby-2.1.2@foss/gems/activerecord-4.1.1/lib/active_record/relation/predicate_builder.rb:10:in `each'
    /Users/pboling/.rvm/gems/ruby-2.1.2@foss/gems/activerecord-4.1.1/lib/active_record/relation/predicate_builder.rb:10:in `grep'
    /Users/pboling/.rvm/gems/ruby-2.1.2@foss/gems/activerecord-4.1.1/lib/active_record/relation/predicate_builder.rb:10:in `resolve_column_aliases'
    /Users/pboling/.rvm/gems/ruby-2.1.2@foss/gems/activerecord-4.1.1/lib/active_record/relation/query_methods.rb:931:in `build_where'
    /Users/pboling/.rvm/gems/ruby-2.1.2@foss/gems/activerecord-4.1.1/lib/active_record/relation/query_methods.rb:566:in `where!'
    /Users/pboling/.rvm/gems/ruby-2.1.2@foss/gems/activerecord-4.1.1/lib/active_record/relation/query_methods.rb:556:in `where'
    /Users/pboling/Documents/trumaker/postgres_ext/test/queries/common_table_expression_test.rb:36:in `block (3 levels) in <top (required)>'

52 runs, 111 assertions, 0 failures, 1 errors, 1 skips
```

Instructions for running tests against all supported versions of AR have been added to README.md.