Fix order of checks for ramsey_constraints

The file Jung2005_Occbinding.mod triggers

Starting Dynare (version 5.1). 
Calling Dynare with arguments: none
Starting preprocessing of the model file ...
ramsey_constraints: can only be used with ramsey_model or ramsey_policy
Ramsey Problem: added 3 Multipliers.

Apparently the check for the presence of ramsey_model triggered by ramsey_constraints is done before evaluating the whole file.