Force the script to run with bash (not sh)

since the generated script(s) kinda assume bash rather than dash, as is
the case on *buntu.  Also saving scripts in ~/.travis/.build by default,
but only for forensic purposes, not using as a cache or whatever.
Merge pull request #204 from meatballhat/run-script-explicitly-with-bash-plus-default-saving

Force the script to run with bash (not sh)