installing: explicit hamlet dependency for more robust building

hledger.cabal

@@ -112,6 +112,7 @@ executable hledger
     build-depends:
                   io-storage >= 0.3 && < 0.4
                  ,yesod >= 0.5.0.3 && < 0.6
+                 ,hamlet == 0.5.*
                  ,convertible-text >= 0.3.0.1 && < 0.4
                  ,data-object >= 0.3.1.2 && < 0.4
                  ,failure >= 0.1 && < 0.2