Atdgen requires biniou version 1.0.6 or later.

Compiling atdgen against biniou 1.0.5 fails with the following:

```
File "ag_ob_run.ml", line 145, characters 11-28:
Error: Unbound value Bi_io.float32_tag
Hint: Did you mean float64_tag?
Makefile:135: recipe for target 'ag_ob_run.cmi' failed
make[1]: *** [ag_ob_run.cmi] Error 2
```

/cc @mjambon