Force jsonschema dependency to 2.6.0

This is to fix this issue when starting the (frozen) application on Windows:

```
  File "C:\Python36-x64\lib\site-packages\jsonschema\validators.py", line 505, in <module>
  File "C:\Python36-x64\lib\site-packages\jsonschema\_utils.py", line 57, in load_schema
  File "C:\Python36-x64\lib\pkgutil.py", line 634, in get_data
OSError: [Errno 34] Result too large: 'jsonschema\\schemas\\draft6.json'
```

requirements.txt

@@ -1,4 +1,4 @@
-jsonschema>=2.4.0
+jsonschema==2.6.0
 aiohttp>=2.3.3,<2.4.0 # pyup: ignore
 aiohttp-cors>=0.5.3,<0.6.0 # pyup: ignore
 yarl>=0.11
@@ -8,4 +8,4 @@ psutil>=3.0.0
 zipstream>=1.1.4
 typing>=3.5.3.0 # Otherwise yarl fails with python 3.4
 prompt-toolkit==1.0.15
-async-timeout<3.0.0 # pyup: ignore; 3.0 drops support for python 3.4
+async-timeout<3.0.0 # pyup: ignore; 3.0 drops support for python 3.4