Update runner_ulimit_warn to a reasonable number

This updates the default runner_ulimit_warn from 4096 to 65536, which is much closer to what we recommend (very high or unlimited). Currently, anyone setting to the recommended 4096 is going to have a bad time.

Changes this:

```
!!!!
!!!! WARNING: ulimit -n is 1024; 4096 is the recommended minimum.
!!!!
```

To this:

```
!!!!
!!!! WARNING: ulimit -n is 1024; 65536 is the recommended minimum.
!!!!
```

Btw, this is prime bikeshedding material.

I'll add this into basho/riak_ee after some +1s