Fix bug when sysctl is not available

[pedrib]

This fixes a bug that happens (at least on my machine) when sysctl is not executable by a non-root user.
Without it, this happens:

autochrome > ruby autochrome.rb -d autochrome_install/
[???] Detected Linux (x86_64)
Traceback (most recent call last):
        7: from autochrome.rb:90:in `<main>'
        6: from autochrome.rb:90:in `new'
        5: from tools/autochrome/lib/auto_chrome.rb:107:in `initialize'
        4: from tools/autochrome/lib/processor/chrome.rb:49:in `new_from_type'
        3: from tools/autochrome/lib/processor/chrome.rb:49:in `new'
        2: from tools/autochrome/lib/processor/unix.rb:9:in `initialize'
        1: from tools/autochrome/lib/processor/linux.rb:14:in `sanity_check'
tools/autochrome/lib/processor/linux.rb:14:in ``': No such file or directory - sysctl (Errno::ENOENT)

The fix is to surround the sysctl command with a begin / rescue statement for Errno::ENOENT.