Downgrade thor to 0.19.1

Version 0.19.4 prints errors to the console, which can be weird for
podmins.

See erikhuda/thor#538

Gemfile.lock

@@ -831,7 +831,7 @@ GEM
       unicode-display_width (~> 1.1.1)
     test_after_commit (1.1.0)
       activerecord (>= 3.2)
-    thor (0.19.4)
+    thor (0.19.1)
     thread_safe (0.3.5)
     tilt (1.4.1)
     timecop (0.8.1)