Fix Readline check on Ruby 1.8.7

`Kernel.const_defined?` will return true for root-level constants in Ruby
1.9+, but false in Ruby 1.8. `Object.const_defined?` works consistently in
all versions.

Author: georgebrock

lib/thor/line_editor/readline.rb

@@ -7,7 +7,7 @@ class Thor
   module LineEditor
     class Readline < Basic
       def self.available?
-        Kernel.const_defined?(:Readline)
+        Object.const_defined?(:Readline)
       end
 
       def readline

spec/line_editor/readline_spec.rb

@@ -10,12 +10,12 @@
 
   describe '.available?' do
     it 'returns true when ::Readline exists' do
-      allow(Kernel).to receive(:const_defined?).with(:Readline).and_return(true)
+      allow(Object).to receive(:const_defined?).with(:Readline).and_return(true)
       expect(described_class).to be_available
     end
 
     it 'returns false when ::Readline does not exist' do
-      allow(Kernel).to receive(:const_defined?).with(:Readline).and_return(false)
+      allow(Object).to receive(:const_defined?).with(:Readline).and_return(false)
       expect(described_class).not_to be_available
     end
   end