Update to use go modules (#671)

This includes updates to CI config, vendor files, etc.
I've hard-coded the k8s depedency at 1.13.4 to keep it inline with what
we currently have and to make sure a another run of `go mod tidy`
doesn't accidentally update it to an unexpected version.

Thanks to hectorj2f for carrying this along.

vendor/golang.org/x/sys/unix/gccgo_c.c

@@ -3,6 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // +build gccgo
+// +build !aix
 
 #include <errno.h>
 #include <stdint.h>
@@ -36,12 +37,3 @@ gccgoRealSyscallNoError(uintptr_t trap, uintptr_t a1, uintptr_t a2, uintptr_t a3
 {
 	return syscall(trap, a1, a2, a3, a4, a5, a6, a7, a8, a9);
 }
-
-// Define the use function in C so that it is not inlined.
-
-extern void use(void *) __asm__ (GOSYM_PREFIX GOPKGPATH ".use") __attribute__((noinline));
-
-void
-use(void *p __attribute__ ((unused)))
-{
-}