Drop graft alias and join_type arguments

JoinDependency#graft was dopped in activerecord and so was passing
join_type when constructing the nodes. See

https://github.com/rails/rails/commit/fadb6d923beb5a45650e6bb73c238538917c25e4
https://github.com/rails/rails/commit/ff5f328bdfdd3dffd31bd74adac9d1e6e7f241fd

This should at least let us load the code without things blow on start
up
Update specs and method signature in JoinAssociation
Remove a couple more deprecated JoinDependency api

Looking into the history for ActiveRecord::Associations::JoinDependency
should give us some help on why each one of those were removed. See for
example:

https://github.com/rails/rails/commit/868457488d3c43ff4e9c28e34883620d90345185
JoinDependency#build_with_polymorphism possibly works again

Looking at both +initialize+ and +build+ methods diff between Rails
4.0.2 and 4.1.0.beta1 should give some reasoning to the changes here.

It looks like all logic previously placed in JoinDependecy#build was
moved into some sort of tree so that all JoinDependecy#build does now is
find the reflection and build the JoinAssociation for that. Bad side
effect here is that JoinDependency#build_with_polymorphism overrides
rails ActiveRecord::JoinDependency#build completely.

https://github.com/rails/rails/blob/v4.0.2/activerecord/lib/active_record/associations/join_dependency.rb#L31-40
https://github.com/rails/rails/blob/v4.0.2/activerecord/lib/active_record/associations/join_dependency.rb#L128-154

https://github.com/rails/rails/blob/v4.1.0.beta1/activerecord/lib/active_record/associations/join_dependency.rb#L95-101
https://github.com/rails/rails/blob/v4.1.0.beta1/activerecord/lib/active_record/associations/join_dependency.rb#L214-225
Drop join_type examples in join_dependecy_spec

The accessor has been deprecated in rails core
JoinAssociation#build_constraint takes klass as first argument

See https://github.com/rails/rails/commit/743b67508e2027e1d086142ccbec47a19fc943f6
Add ActiveRecord namespace in EagerLoadPolymorphicError
Gemfile: use https for rubygems and Ruby 1.9+ hash syntax
Update the README and mention the new Rails-4.1 branch
Support both activerecord 4.1 and below

Still need to update specs to run on activerecord <= 4.0
Bump rspec to 2.14.0
Separate specs per activerecord version
Remove arel from Gemfile

Not really sure why it was there, activerecord seems to load arel
already. Plus it was giving some conflicts when running the build on
rails versions other than master (you needed to define both RAILS and
AREL env vars)
Add Rails 4.1 support to master while maintaining Rails 3.x and 4.0 compatibility