Upgrade thor to 0.20.1

Gemfile

@@ -104,8 +104,6 @@ group :test do
   platforms :mri do
     gem "stackprof"
     gem "byebug"
-    # FIXME: Remove this when thor 0.21 is release
-    gem "thor", git: "https://github.com/erikhuda/thor.git", ref: "006832ea32480618791f89bb7d9e67b22fc814b9"
   end
 
   gem "benchmark-ips"

Gemfile.lock

@@ -1,10 +1,3 @@
-GIT
-  remote: https://github.com/erikhuda/thor.git
-  revision: 006832ea32480618791f89bb7d9e67b22fc814b9
-  ref: 006832ea32480618791f89bb7d9e67b22fc814b9
-  specs:
-    thor (0.20.0)
-
 GIT
   remote: https://github.com/matthewd/rb-inotify.git
   revision: 856730aad4b285969e8dd621e44808a7c5af4242
@@ -477,6 +470,7 @@ GEM
       daemons (~> 1.0, >= 1.0.9)
       eventmachine (~> 1.0, >= 1.0.4)
       rack (>= 1, < 3)
+    thor (0.20.1)
     thread_safe (0.3.6)
     thread_safe (0.3.6-java)
     tilt (2.0.8)
@@ -569,7 +563,6 @@ DEPENDENCIES
   sqlite3 (~> 1.3.6)
   stackprof
   sucker_punch
-  thor!
   turbolinks (~> 5)
   tzinfo-data
   uglifier (>= 1.3.0)