Update tests using free for procps-ng >= 3.3.10
Update tests using free for procps-ng >= 3.3.10

procps-ng 3.3.10 changed the output format of free
and removed the 'buffers/cache' line.
Update the tests which use free to be compatible with
this new version and for buffers/cache tests use vmstat.

References: #538

mrjefftang/psutil,giampaolo/psutil,mrjefftang/psutil,0-wiz-0/psutil,0-wiz-0/psutil,giampaolo/psutil,mrjefftang/psutil