fix(docs): Pinned Sphinx to 1.3.6 to make sure build passes
fix(docs): Pinned Sphinx to 1.3.6 to make sure build passes

Without this, the following error occurs:

```
Warning, treated as error:
simpl/docs/source/simpl.rst:19: WARNING: Could not lex literal_block as
"python". Highlighting skipped.
```

larsbutler/simpl,checkmate/simpl