Allow dev versions of CWL

Otherwise do_update (which we want to update from older CWL versions)
will update tools to the latest version, which may be a dev one,
causing errors like:

```
schema_salad.exceptions.ValidationException: Version 'v1.3.0-dev1' is a development or deprecated version.
 Update your document to a stable version (v1.0, v1.1, v1.2) or use --enable-dev to enable support for development and deprecated versions.
```

Together with the fixes to not unnecessarily recreate ToolProxy, fix
common-workflow-lab#147

lib/galaxy/tool_util/cwl/schema.py

@@ -44,6 +44,7 @@ def loading_context(self):
         loading_context.strict = self._strict
         loading_context.do_validate = self._validate
         loading_context.loader = self.raw_document_loader
+        loading_context.enable_dev = True
         loading_context.do_update = True
         loading_context.relax_path_checks = True
         return loading_context