Exchange debugger for byebug

https://github.com/cldwalker/debugger/issues/125#issuecomment-43353446

Gemfile

@@ -131,5 +131,5 @@ group :test do
 end
 
 group :development, :test do
-  gem 'debugger'
+  gem 'byebug'
 end

Gemfile.lock

@@ -61,6 +61,9 @@ GEM
       sass (~> 3.2)
     browser (0.6.0)
     builder (3.2.2)
+    byebug (3.1.2)
+      columnize (~> 0.8)
+      debugger-linecache (~> 1.2)
     cancan (1.6.10)
     capybara (2.2.1)
       mime-types (>= 1.16)
@@ -100,12 +103,7 @@ GEM
     d3_rails (3.4.6)
       railties (>= 3.1.0)
     database_cleaner (1.3.0)
-    debugger (1.6.8)
-      columnize (>= 0.3.1)
-      debugger-linecache (~> 1.2.0)
-      debugger-ruby_core_source (~> 1.3.5)
     debugger-linecache (1.2.0)
-    debugger-ruby_core_source (1.3.5)
     devise (3.2.4)
       bcrypt (~> 3.0)
       orm_adapter (~> 0.1)
@@ -400,6 +398,7 @@ DEPENDENCIES
   awesome_nested_set (~> 3.0.0.rc.5)
   axlsx_rails
   bootstrap-sass
+  byebug
   cancan
   capybara
   chart-js-rails
@@ -407,7 +406,6 @@ DEPENDENCIES
   coveralls
   d3_rails
   database_cleaner
-  debugger
   devise
   factory_girl_rails
   font-awesome-rails