Use sysctl hw.memsize instead of sysctl -a | grep

The usage of `sysctl hw.memsize` saves us the call of grep.

Also (at least on Yosemite), `sysctl -a` causes a `kernel: AudioSysctl: (0)` message to be written to the system logs on each widget update.

Cheers and thank you for this beautiful widget!
:octocat: Jérôme