Fixed silent keyword arg to config.from_envvar.

The ``silent`` keyword argument to Config.from_envvar was not being honored
if the environment variable existed but the file that it mentioned did not.
The fix was simple - pass the keyword argument on to the underlying call to
``from_pyfile``.  I also noticed that the return value from ``from_pyfile``
was not being passed back so I fixed that as well.

flask/config.py

@@ -106,8 +106,7 @@ class Config(dict):
                                'loaded.  Set this variable and make it '
                                'point to a configuration file' %
                                variable_name)
-        self.from_pyfile(rv)
-        return True
+        return self.from_pyfile(rv, silent=silent)
 
     def from_pyfile(self, filename, silent=False):
         """Updates the values in the config from a Python file.  This function