Update Yesod.hs

Fix elm-yesod to compile against yesod 1.1.7.1 (shakespeare 1.1.1).
Broken by "Disable legacy ToJavascript instances" change to julius 1.0.2 -> 1.1.0
See:
https://github.com/yesodweb/shakespeare/commit/d5a1b570198d79becee71f5a06fe3e45fba8a902#shakespeare-js/Text/Julius.hs

Fixes issues 1 and 2
Merge pull request #4 from ngunn/master

Fix issues #2 and #3 (making elm-yesod compile against yesod 1.1.7.1)