Maintain 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.