Subtract inactive_file from memory usage

Depending on which way you want to go with story [69951118](https://www.pivotaltracker.com/story/show/69951118), this implements the change to remove the `inactive_file` portion of the cache from the reported memory usage statistic.  The most notable impact is that the usage reported immediately after start went from ~70M to about ~22M with a simple ruby app.

For the test listed in the story, I actually created a 500M file in the container after pushing the app.  The memory usage reported went from 22.0M to 22.7M even though the cgroup's memory.usage_in_bytes statistic went from 73740288 bytes to 266739712 bytes.  (The container's memory limit was 256M.)

Please see the mailing list thread I pointed out in tracker for some of the discussion that occurred when I initially made this proposal.