MB-100: Make _drop_old_crashdumps work on OS X

The function _drop_old_crashdumps passes negative number to
head -n argument. This is supported only by GNU head and not
others, specifically OS X. Requires bash.

Change-Id: Id14f4618970632f38a21a204dc3f74db19c2e999
Reviewed-on: http://review.couchbase.org/32552
Tested-by: Sriram Melkote <siri@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>