try to fix bug in some configs

Usually Pandoc returns the key 'pandoc-api-version' first:

```
>>> echo "a" | pandoc --to=json

{"pandoc-api-version":[1,17,0,4],"meta":{},"blocks":[{"t":"Para","c":[{"t":"Str","c":"\"a\""}]}]}
```

But this is not the case in some systems (which ones?):

```
cat pftest.md | pandoc -t json
{"blocks":[{"t":"Para","c":[{"t":"Str","c":"From"}... ,
],"pandoc-api-version":[1,17,0,4],"meta":{"author":{ ....
```

This workaround should fix that problem, but if the key order is
different at other places that might create future issues.

panflute/elements.py

@@ -1338,13 +1338,16 @@ def from_json(data):
         return c
 
     # Document (New API)
-    if data[0][0] == 'pandoc-api-version':
-            assert len(data) == 3
-            assert data[1][0] == 'meta'
-            assert data[2][0] == 'blocks'
-            api, meta, items = (pair[1] for pair in data)
-            doc = Doc(*items, api_version=api, metadata=meta)
-            return doc
+    if len(data) == 3:
+        # It seems that (in some configs or systems),
+        # the first element might not always be "pandoc-api-version"
+        if data[0][0] in ('pandoc-api-version', 'meta', 'blocks'):
+            data = OrderedDict(data)
+            api = data['pandoc-api-version']
+            meta = data['meta']
+            items = data['blocks']
+
+            return Doc(*items, api_version=api, metadata=meta)
 
     # Metadata contents
     if data[0][0] != 't':

setup.py

@@ -21,7 +21,7 @@
     # Versions should comply with PEP440.  For a discussion on single-sourcing
     # the version across setup.py and the project code, see
     # https://packaging.python.org/en/latest/single_source_version.html
-    version='1.4.0',
+    version='1.4.1',
 
     description='Pythonic Pandoc filters',
     long_description=long_description,