Tidy the Travis config, build on Arm

[peternewman]

• Changing to bionic will be blocking on plugins/convert_README_to_header.sh doesn't work in bionic (and possibly others after xenial) #1592 in master, Simplify the sed command used to generate plugin header files, closes #1592 (hopefully) #1608 has covered the basics.
• Try changing back to python-numpy (deb rather than pip install)
• see if pychecker can find the module now when done via deb