It's not support memory info from 'free of procps-ng'

[yuzhigang33]

Hi, @timjrobinson
The current code is assuming free command in linux is from procps, but in some linux the free command is from procps-ng, and the two command's output are not the same. see bellow:

procps-ng 's output:

              total        used        free      shared  buff/cache   available
Mem:          96359        6734       11380           4       78244       89198
Swap:          2047         155        1892

procps 's output:

             total       used       free     shared    buffers     cached
Mem:         96359      84976      11382          0       4293      68691
-/+ buffers/cache:      11991      84367
Swap:         2047        155       1892

So could you update it to support the two scenes ?