Fix aix memory plugin output

[sh9189]

Memory plugin does not return any memory attributes on aix due to a minor bug in definition of memory Mash. This pull request is to fix that and also adds unit tests for aix memory plugin

[johnbellone]

👍 from our end here.

[irvingpop]

Paging @scotthain for review

[irvingpop]

The reason this was broken in ohai was because there's no memory test for AIX - this PR adds it.

[scotthain]

👍

[irvingpop]

Travis is acting funny, but I can confirm via rspec that the new memory test works:

Ohai::System AIX memory plugin
  should get total memory
  should get free memory