[PodSourcePreparer] Don't leak version var on failure

lib/cocoapods/installer/pod_source_preparer.rb

@@ -58,15 +58,18 @@ module Pod
         return unless spec.prepare_command
         UI.section(' > Running prepare command', '', 1) do
           Dir.chdir(path) do
+            begin
               ENV.delete('CDPATH')
               ENV['COCOAPODS_VERSION'] = Pod::VERSION
               prepare_command = spec.prepare_command.strip_heredoc.chomp
               full_command = "\nset -e\n" + prepare_command
               bash!('-c', full_command)
+            ensure
               ENV.delete('COCOAPODS_VERSION')
             end
           end
         end
+      end
 
       #-----------------------------------------------------------------------#
     end

spec/unit/installer/pod_source_installer_spec.rb

@@ -63,6 +63,13 @@ module Pod
           @spec.prepare_command = "[ \"$COCOAPODS_VERSION\" == \"#{Pod::VERSION}\" ] || exit 1"
           lambda { @installer.install! }.should.not.raise
         end
+
+        it 'doesn\'t leak the $COCOAPODS_VERSION environment variable' do
+          ENV['COCOAPODS_VERSION'] = nil
+          @spec.prepare_command = 'exit 1'
+          lambda { @installer.install! }.should.raise(Pod::Informative)
+          ENV['COCOAPODS_VERSION'].should.be.nil
+        end
       end
 
       #--------------------------------------#