[Analyzer] Use target_definition.empty? instead of dependencies

Use the more abstract `empty?` method. It excludes inherited
dependencies which is enough for this validation.
Add test where the target inherits the platform from its parent to
ensure the validation passes in this case as well.

lib/cocoapods/installer/analyzer.rb

@@ -636,7 +636,7 @@ module Pod
       def verify_platforms_specified!
         unless config.integrate_targets?
           podfile.target_definition_list.each do |target_definition|
-            if target_definition.dependencies.size > 0 && target_definition.platform.nil?
+            if !target_definition.empty? && target_definition.platform.nil?
               raise Informative, 'It is necessary to specify the platform in the Podfile if not integrating.'
             end
           end

spec/unit/installer/analyzer_spec.rb

@@ -221,7 +221,7 @@ module Pod
           config.integrate_targets = false
         end
 
-        it 'does not require a platform for a podless target' do
+        it 'does not require a platform for an empty target' do
           podfile = Pod::Podfile.new do
             source SpecHelper.test_repo_url
             xcodeproj 'SampleProject/SampleProject'
@@ -235,7 +235,21 @@ module Pod
           lambda { analyzer.analyze }.should.not.raise
         end
 
-        it 'raises if a target with pods does not specify a platform' do
+        it 'does not raise if a target with dependencies inherits the platform from its parent' do
+          podfile = Pod::Podfile.new do
+            source SpecHelper.test_repo_url
+            xcodeproj 'SampleProject/SampleProject'
+            platform :osx
+            target 'TestRunner' do
+              pod 'monkey'
+            end
+          end
+
+          analyzer = Pod::Installer::Analyzer.new(config.sandbox, podfile)
+          lambda { analyzer.analyze }.should.not.raise
+        end
+
+        it 'raises if a target with dependencies does not have a platform' do
           podfile = Pod::Podfile.new do
             source SpecHelper.test_repo_url
             xcodeproj 'SampleProject/SampleProject'