Fix parsing of ++ operator

Dhall was failing to parse `++` because the `+` operator was being parsed with
higher priority.  The fix is to switch the order in which the two operator
parses are tried

src/Dhall/Parser.hs

@@ -293,8 +293,8 @@ exprC = expressionParser
           , operator Combine       combine
           ]
         , [ operator BoolOr       (symbol "||")
-          , operator NaturalPlus  (symbol "+" )
           , operator TextAppend   (symbol "++")
+          , operator NaturalPlus  (symbol "+" )
           ]
         , [ operator BoolEQ       (symbol "==")
           , operator BoolNE       (symbol "/=")