bump mixlib-shellout deps

the encoding changes that prompted mixlib-shellout to bump to 2.0 should
not affect test-kitchen, so leave the bottom version at 1.2 and raise
the top constraint to 2.x
Merge pull request #531 from lamont-granquist/lcg/mixlib-shellout

bump mixlib-shellout deps
Clarify comments in configuration loader.

The original comment used words like "higher" to mean "physically above
another element" and "lower" to mean "physically below another element"
but the list was numbered in increasing order, so item 3 was 'lower'
than item 2 (isn't that confusing when 3 is lower than 2, but 3 is
greater than 2 numerically). This is extremely counterintuitive for
people who assume "higher" means "numerically larger/greater", and
"lower" means "numerically smaller/less."
Merge pull request #507 (rebased) from martinb3/master

Closes #507