[Specification] Return lowest supported deployment target if needed.

Closes #357

lib/cocoapods/specification.rb

@@ -474,7 +474,27 @@ module Pod
     end
 
     def deployment_target(platform)
-      @deployment_target[platform] || ( @parent ? @parent.deployment_target(platform) : nil )
+      @deployment_target[platform] || ( @parent ? @parent.deployment_target(platform) : lowest_supported_deployment_target(platform) )
+    end
+
+    # Returns the lowest supported deployment target for a given platform.
+    #   The value should be pegged to the lowest deployment target supported
+    #   by a default xcode installation.
+    #
+    # @param platform [Symbol] The platform for witch the deployment target
+    #   shoud be returned.
+    #
+    # @return [String] The deployment target.
+    #
+    def lowest_supported_deployment_target(platform)
+      case platform
+      when :ios
+        '3.0'
+      when :osx
+        '10.4'
+      else
+        nil
+      end
     end
   end
   Spec = Specification

spec/unit/specification_spec.rb

@@ -552,7 +552,7 @@ describe "A Pod::Specification, concerning its attributes that support different
 
     it "returns the list of the supported platfroms and deployment targets" do
      @spec.available_platforms.count.should == 2
-     @spec.available_platforms.should.include? Pod::Platform.new(:osx)
+     @spec.available_platforms.should.include? Pod::Platform.new(:osx, '10.4')
      @spec.available_platforms.should.include? Pod::Platform.new(:ios, '4.0')
     end