Clean up script commands in .travis.yml.

Besides general whitespace and quoting cleanup, commands that test
variables were changed from:
  [ $B = a ] && command || true
to
  if [ $B = a ] ; then command ; fi
This form correctly returns the exit code of "command".
Add retries to commands that download from external servers.

This helps reduce the chance of a build failure due to transient
download error.  This also switches to consistent use of wget
throughout, which reports download errors more cleanly in the travis
build logs.
Rename travis "arm" BUILD_ARCH to "arm-apcs".

These platforms are built with an old ARM compiler that supports the
deprecated APCS standard.
Increase fixed filename sizes in SLIP tunnels

Long serial port names like
  /dev/serial/by-id/usb-FTDI_FT230X_Basic_UART_DN0038W8-if00-port0
cause crashes.  This is the simplest fix to avoid the problem.
Merge pull request #748 from jimparis/travis-cleanup

Travis cleanup
Merge pull request #737 from jimparis/slip-buffer-overflow-fixes

Increase fixed filename sizes in SLIP tunnels
Merge commit '01bd045570ee3552aa103cc65baeb78ad70240c1' into HEAD