Update ruby debugger gem to be compatible with Ruby 1.9, 2.0, and 2.1

Gemfile.lock

@@ -7,12 +7,12 @@ GEM
     columnize (0.3.6)
     commander (4.1.3)
       highline (~> 1.6.11)
-    debugger (1.6.0)
+    debugger (1.6.5)
       columnize (>= 0.3.1)
       debugger-linecache (~> 1.2.0)
-      debugger-ruby_core_source (~> 1.2.1)
+      debugger-ruby_core_source (~> 1.3.1)
     debugger-linecache (1.2.0)
-    debugger-ruby_core_source (1.2.2)
+    debugger-ruby_core_source (1.3.1)
     directory_watcher (1.4.1)
     fast-stemmer (1.0.2)
     highline (1.6.19)