nsswitch: Add try_authok option to pam_winbind

[github at samba.org]

New comment by sathieu on Samba Github repository

https://github.com/samba-team/samba/pull/168#issuecomment-395348148
Comment:
This PR includes a fix for travis. Please merge `autobuild: Fix travis build (broken since ldb lmdb backend)`: https://github.com/samba-team/samba/pull/168/commits/939c94832e722021222c4c145ce58406a0d23288