make mapconv easier to compile

i've added squish to the repo as it doesn't exist on ubuntu. still a system installed squish is prefered, if installed.

sadly it doesn't compile yet:

/home/travis/build/abma/MapConv/src/smtool.cpp:216:9: error: ‘cut’ is not a member of ‘OpenImageIO::v1_3::ImageBufAlgo’

i don't have the time to fix this, but with the addition of .travis.yml it shouldn't break unnoticed any more :-)