Update admin_main_view.php (#578)
Update admin_main_view.php (#578)

- Modify the RAM graph to show RAM in use - Buffers and - Cached; this change gives a better representation of RAM in use by applications.

- Add MQTT version information.

Both modifications suggested and mostly coded by @JonMurphy 

https://github.com/emoncms/emoncms/pull/577

https://community.openenergymonitor.org/t/linux-memory-vs-real-memory/1929/18

Tweak to the Text beneath the Memory Graph to reflect recent changes to the graph.

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