rename variable REQUIRED_LIBS to REQUIRED_PACKAGES

this change is to be more consistent with other CMakelists.txt in the
project.
use cmake find_package to get add jpeg dependency

this was prompted because on mac, simply adding -ljpeg does not work
when libjpeg is installed in /usr/local/lib via homebrew.  In general,
find_package should be used for all of libbots dependencies, but im
only fixing jpeg in bot2-vis for now.
remove jpeg dependency from bot2-frames

the jpeg dependency will be added correctly because of the declared
pkg-config dependency on bot2-vis.
Merge pull request #2 from patmarion/fix-jpeg-link

Fix jpeg link