Remove Rails 3.2 support for now.

It is impossible to run our tests on both Rails 3.2 and Rails 4:

1. Rails 3.2 relies on test/unit, which is MiniTest 4.
2. Rails 4 relies on MiniTest 5.
3. MiniTest 5 is not compatible with test/unit.

:cry: :cry: :cry:

.travis.yml

@@ -7,7 +7,6 @@ rvm:
   - rbx-2
   - ruby-head
 env:
-  - "RAILS_VERSION=3.2"
   - "RAILS_VERSION=4.0"
   - "RAILS_VERSION=4.1"
   - "RAILS_VERSION=master"

active_model_serializers.gemspec

@@ -18,8 +18,8 @@ Gem::Specification.new do |spec|
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
   spec.require_paths = ["lib"]
 
-  spec.add_dependency "activesupport", ">= 3.2"
-  spec.add_dependency "rails", ">= 3.2"
+  spec.add_dependency "activesupport", ">= 4.0"
+  spec.add_dependency "rails", ">= 4.0"
 
   spec.add_development_dependency "bundler", "~> 1.6"
   spec.add_development_dependency "rake"