Add the ability to remove a dependency from the dependency list

lib/appraisal/dependency_list.rb

@@ -4,10 +4,19 @@ module Appraisal
   class DependencyList
     def initialize
       @dependencies = Hash.new
+      @removed_dependencies = Set.new
     end
 
     def add(name, requirements)
-      @dependencies[name] = Dependency.new(name, requirements)
+      unless @removed_dependencies.include?(name)
+        @dependencies[name] = Dependency.new(name, requirements)
+      end
+    end
+
+    def remove(name)
+      if @removed_dependencies.add?(name)
+        @dependencies.delete(name)
+      end
     end
 
     def to_s

spec/appraisal/dependency_list_spec.rb

@@ -28,4 +28,23 @@
       expect(dependency_list.to_s).to eq %(gem "rails", "4.1.4")
     end
   end
+
+  describe "#remove" do
+    let(:dependency_list) { Appraisal::DependencyList.new }
+
+    before do
+      dependency_list.add("rails", ["4.1.4"])
+    end
+
+    it "removes the dependency from the list" do
+      dependency_list.remove("rails")
+      expect(dependency_list.to_s).to eq("")
+    end
+
+    it "respects the removal over an addition" do
+      dependency_list.remove("rails")
+      dependency_list.add("rails", ["4.1.0"])
+      expect(dependency_list.to_s).to eq("")
+    end
+  end
 end