Propagate version to prepare_command

lib/cocoapods/installer/pod_source_preparer.rb

@@ -59,9 +59,11 @@ module Pod
         UI.section(' > Running prepare command', '', 1) do
           Dir.chdir(path) do
             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)
+            ENV.delete('COCOAPODS_VERSION')
           end
         end
       end

spec/unit/installer/pod_source_installer_spec.rb

@@ -58,6 +58,11 @@ module Pod
           @spec.prepare_command = 'cd Classes;ls Banana.h'
           lambda { @installer.install! }.should.not.raise
         end
+
+        it 'sets the $COCOAPODS_VERSION environment variable' do
+          @spec.prepare_command = "[ \"$COCOAPODS_VERSION\" == \"#{Pod::VERSION}\" ] || exit 1"
+          lambda { @installer.install! }.should.not.raise
+        end
       end
 
       #--------------------------------------#