make the gre test pass

.travis.yml

@@ -1,5 +1,8 @@
 language: go
 before_script:
+  # make sure we keep path in tact when we sudo
   - sudo sed -i -e 's/^Defaults\tsecure_path.*$//' /etc/sudoers
+  # modprobe ip_gre or else the first gre device can't be deleted
+  - sudo modprobe ip_gre
 install:
   - go get github.com/vishvananda/netns

link_test.go

@@ -114,20 +114,15 @@ func testLinkAddDel(t *testing.T, link Link) {
 		t.Fatal(err)
 	}
 
-	// It appears that some links don't disappear immediately. Specifically,
-	// the first GreTap device takes a moment to delete.
-	for i := 0; i < 10; i++ {
 	links, err = LinkList()
 	if err != nil {
 		t.Fatal(err)
 	}
 
-		if len(links) == num {
+	if len(links) != num {
+		t.Fatal("Link not removed properly")
 		return
 	}
-		time.Sleep(100 * time.Millisecond)
-	}
-	t.Fatal("Link not removed properly")
 }
 
 func compareVxlan(t *testing.T, expected, actual *Vxlan) {