Don't write translation file when T.is_writable == False

Web2py manual says in [Translating variables section](http://web2py.com/books/default/chapter/29/04/the-core#Translating-variables) one can disable write of translation files to improve performance with this setting:

```T.is_writable = False```

But Web2py 2.9.5 just checks if filesystem is writable, ignoring T.is_writable at all.

This PR fixes this behaviour.