[SourcesManager] Dont return non-functioning master sources

lib/cocoapods/sources_manager.rb

@@ -103,7 +103,7 @@ module Pod
       # @return [Array<Source>] The CocoaPods Master Repo source.
       #
       def master
-        sources(['master'])
+        sources(['master']).select { |s| s.repo.directory? }
       end
 
       # Search the appropriate sources to match the set for the given dependency.

spec/unit/sources_manager_spec.rb

@@ -431,6 +431,11 @@ module Pod
         SourcesManager.master_repo_dir.to_s.should.match %r{fixtures/spec-repos/master}
       end
 
+      it 'returns an empty array for master sources when the master repo has not been set up' do
+        Pathname.any_instance.stubs(:directory?).returns(false)
+        SourcesManager.master.should == []
+      end
+
       it 'returns whether the master repo is functional' do
         SourcesManager.master_repo_functional?.should.be.true
         config.repos_dir = SpecHelper.temporary_directory