Go 1.13 support (#1546)

* 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.

pkg/terminal/command_test.go

@@ -733,6 +733,13 @@ func TestConfig(t *testing.T) {
 
 func TestDisassembleAutogenerated(t *testing.T) {
 	// Executing the 'disassemble' command on autogenerated code should work correctly
+
+	if goversion.VersionAfterOrEqual(runtime.Version(), 1, 13) {
+		// CL 161337 in Go 1.13 and later removes the autogenerated init function
+		// https://go-review.googlesource.com/c/go/+/161337
+		t.Skip("no autogenerated init function in Go 1.13 or later")
+	}
+
 	withTestTerminal("math", t, func(term *FakeTerminal) {
 		term.MustExec("break main.init")
 		term.MustExec("continue")