[Podfile] Check that platforms actually have a deployment target.

Closes #305.

spec/unit/podfile_spec.rb

@@ -251,6 +251,12 @@ describe "Pod::Podfile" do
       @podfile.target_definitions[:osx_target].platform.should == :osx
     end
 
+    it "assigs a deployment target to the platforms if not specified" do
+      @podfile.target_definitions[:default].platform.deployment_target.to_s.should == '4.3'
+      @podfile.target_definitions[:test].platform.deployment_target.to_s.should == '4.3'
+      @podfile.target_definitions[:osx_target].platform.deployment_target.to_s.should == '10.6'
+    end
+
     it "autmatically marks a target as exclusive if the parent platform doesn't match" do
       @podfile.target_definitions[:osx_target].should.be.exclusive
       @podfile.target_definitions[:nested_osx_target].should.not.be.exclusive