Allow Thor 0.15 and 1.0, to be released shortly

railties/railties.gemspec

@@ -21,8 +21,12 @@ Gem::Specification.new do |s|
   s.rdoc_options << '--exclude' << '.'
 
   s.add_dependency('rake',          '>= 0.8.7')
-  s.add_dependency('thor',          '~> 0.14.6')
   s.add_dependency('rack-ssl',      '~> 1.3.2')
+
+  # The current API of the Thor gem (0.14) will remain stable at least until Thor 2.0. Because
+  # Thor is so heavily used by other gems, we will accept Thor's semver guarantee to reduce
+  # the possibility of conflicts.
+  s.add_dependency('thor',          '>= 0.14.6', '< 2.0')
   s.add_dependency('rdoc',          '~> 3.4')
   s.add_dependency('activesupport', version)
   s.add_dependency('actionpack',    version)