Update runtime.txt

```
remote: -----> Python app detected
remote:  !     The latest version of Python 3 is python-3.6.3 (you are using python-3.6.2, which is unsupported).
remote:  !     We recommend upgrading by specifying the latest version (python-3.6.3).
```

runtime.txt

-python-3.6.2
+python-3.6.3