Fix compatibility with Rake 0.9.x.

This method is incredibly unsatisfactory, please see:

 * jimweirich/rake#81

for more discussion about this issue.

lib/capistrano.rb

@@ -1,3 +1,5 @@
+require 'capistrano/fix_rake_deprecated_dsl'
+
 require 'capistrano/configuration'
 require 'capistrano/extensions'
 require 'capistrano/ext/string'

lib/capistrano/fix_rake_deprecated_dsl.rb

@@ -0,0 +1,8 @@
+#
+# See https://github.com/jimweirich/rake/issues/81
+#
+if defined?(Rake::DeprecatedObjectDSL)
+  Rake::DeprecatedObjectDSL.private_instance_methods.each do |m|
+    Rake::DeprecatedObjectDSL.send("undef_method", m)
+  end
+end