Upgrade debugger version

This is failing in travisci, I assume it's because it's using an older
version

Author: zmoazeni

Gemfile.lock

@@ -17,12 +17,12 @@ GEM
       chunky_png (~> 1.2)
       fssm (>= 0.2.7)
       sass (~> 3.1)
-    debugger (1.5.0)
+    debugger (1.6.0)
       columnize (>= 0.3.1)
       debugger-linecache (~> 1.2.0)
-      debugger-ruby_core_source (~> 1.2.0)
+      debugger-ruby_core_source (~> 1.2.1)
     debugger-linecache (1.2.0)
-    debugger-ruby_core_source (1.2.0)
+    debugger-ruby_core_source (1.2.2)
     fssm (0.2.10)
     less (2.3.1)
       commonjs (~> 0.2.6)