* 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.
Before doing anything check that the version of Go is compatible with
the current version of Delve.
This will improve the error message in the case that another change as
disruptive as Go1.11 dwarf compression, happens.
