travis: turn off libmount on OSX

travis: turn off libmount on OSX

Now that the absence of the sscanf modifiers no longer prevents Travis from building libmount
automatically it seems util-linux is failing to compile there with
```
In file included from disk-utils/fsck.c:49:

[1m./libmount/src/libmount.h:32:10: [0m[0;1;31mfatal error: [0m[1m'mntent.h' file not found[0m

[0;1;32m         ^~~~~~~~~~

[0m1 error generated.

make[2]: *** [disk-utils/fsck-fsck.o] Error 1

make[2]: *** Waiting for unfinished jobs....

make[1]: *** [all-recursive] Error 1

make: *** [all] Error 2
```

Signed-off-by: Evgeny Vereshchagin <evvers@ya.ru>