tmp travis: use related opam-rt branch
curl: fail on error
Parallelise dev package update
Update: better display
Remove the 'packages UPDATED' messages unless in verbose mode

they're not that useful since we get the update summary, have been proved confusing
on occasions and don't carry much worthwile information
verbose mode: display command output at end, don't call tee
Be more verbose on compiler download and install
Downloads: for consistency, display message when done