Bump coffee-script-source to fix bug

Coffee script 1.12.1 was released and contained a bug where coffee
script was undefined. 1.12.1 was yanked and 1.12.2 was released. This
bumps coffee-script-source so that CI Railties tests pass.

See: jashkenas/coffeescript#4403

Gemfile.lock

@@ -162,7 +162,7 @@ GEM
     coffee-script (2.4.1)
       coffee-script-source
       execjs
-    coffee-script-source (1.10.0)
+    coffee-script-source (1.12.2)
     concurrent-ruby (1.0.2)
     connection_pool (2.2.0)
     cookiejar (0.3.3)