Fixed an incompatibility with Ruby 1.9.

Ruby 1.8 strings are Enumerable, but there is no String#lines method. In
Ruby 1.9, the situation is reversed.  To work around this disparity, the
RailsEnvironment#externals method now explicitly checks whether a String
responds_to? :lines.

[#2130 state:committed]

Signed-off-by: Jeremy Kemper <jeremy@bitsweat.net>

railties/lib/commands/plugin.rb

@@ -134,7 +134,8 @@ class RailsEnvironment
   def externals
     return [] unless use_externals?
     ext = `svn propget svn:externals "#{root}/vendor/plugins"`
-    ext.reject{ |line| line.strip == '' }.map do |line| 
+    lines = ext.respond_to?(:lines) ? ext.lines : ext
+    lines.reject{ |line| line.strip == '' }.map do |line|
       line.strip.split(/\s+/, 2) 
     end
   end