Fix reported Rails problem with rake rails:update [#170 state:resolved]

see http://stackoverflow.com/questions/1218977/when-i-run-any-rake-command-i-get-uninitialized-constant-gemspecification

CHANGELOG

@@ -1,5 +1,6 @@
 === Edge
 
+* Fix uninitialized constant Gem::Specification::PLATFORM_CROSS_TARGETS in Rails [Jason Garber]
 * Allow uppercase letters in ID attribute even though it's invalid [Jason Garber]
 * Fix compatibility with newer Echoe, by using full-name for Platform [Flameeyes]
 * Fixes for PPC/PPC64 [Flameeyes]

lib/tasks/pureruby.rake

@@ -1,12 +1,17 @@
-Gem::Specification::PLATFORM_CROSS_TARGETS << "pureruby"
+# Apparently this file gets loaded by Rails. Only want to define the pureruby
+# task in the context of RedCloth compilation (echoe loaded).
 
-task 'pureruby' do
-  reset_target 'pureruby'
-end
+if Gem::Specification.const_defined?(:PLATFORM_CROSS_TARGETS)
+  Gem::Specification::PLATFORM_CROSS_TARGETS << "pureruby"
 
-if target = ARGV.detect do |arg| 
-  # Hack to get the platform set before the Rakefile evaluates
-    Gem::Specification::PLATFORM_CROSS_TARGETS.include? arg
+  task 'pureruby' do
+    reset_target 'pureruby'
   end
-  reset_target target
-end
+
+  if target = ARGV.detect do |arg| 
+    # Hack to get the platform set before the Rakefile evaluates
+      Gem::Specification::PLATFORM_CROSS_TARGETS.include? arg
+    end
+    reset_target target
+  end
+end