Skip -printf checks in Govet

Due to https://github.com/golang/go/issues/12294, our Travis jobs are
failing with

    pkg/compiler/binder/stmtexpr.go:160: no formatting directive in Errorf call

etc, etc.  Perplexingly, this is supposedly fixed as of Go 1.7.5, and we are
running Go 1.8.2 in Travis.  However, vet is a separate binary tool, so it seems
either (1) Go's Linux distro doesn't contain the right version or (2) Travis's
image doesn't contain the right version.  I have to imagine (2) is more likely,
except that I can't actually look at the machine (and go vet doesn't have a version)!

For now, we will just skip the -printf checks.

Makefile

@@ -35,5 +35,5 @@ test:
 
 vet:
 	@echo "\033[0;32mVET:\033[0m"
-	@go vet ${PROJECT_PKGS}
+	@go tool vet -printf=false cmd/ lib/ pkg/