Asset builds should include vmstat gem

[cwjohnston]

Per the readme, check-ram.rb requires vmstat but that gem is not part of this gem's runtime requirements (not installed by default).

In order to make check-ram.rb functional for those consuming releases as Sensu Go asset builds, we need to explicitly install the vmstat gem as part of the build process.

[jsnod]

Any update on this @jspaleta ? I am unable to use this plugin in my Sensu Go 6.2.0 setup because of the missing vmstat gem.