* tests: fix tests for Go 1.13
- Go 1.13 doesn't autogenerate init functions anymore, tests that
expected that now fail and should be skipped.
- Plugin tests now need -gcflags'all=-N -l' now, we were probably
getting lucky with -gcflags='-N -l' before.
* proc: allow signed integers as shift counts
Go1.13 allows signed integers to be used as the right hand side of a
shift operator, change eval to match.
* goversion: update maximum supported version
* travis: force Go to use vendor directory
Travis scripts get confused by "go: downloading" lines, the exact
reason is not clear. Testing that the vendor directory is up to date is
a good idea anyway.
Currently Travis supports only Ubuntu 12.04 linux 2.6 VMs which are
terribly outdated, and trying to track down why it's hanging on Travis
but not on other similar VMs is more trouble than it's worth at the
moment.
Certainly another more appropriate and up to date CI solution should be
explored.
