Enable Windows on CI

This ensures that the package compiles on Windows, and that ErrNotSupported is returned.
We need to remove testing on Go master, since this is currently broken on Travis. Testing
master doesn't make much sense in the first place.

.travis.yml

@@ -3,8 +3,8 @@ language: go
 go:
   - "1.13.x"
   - "1.14.x"
-  - master
 
 os:
   - linux
   - osx
+  - windows

upgrader_test.go

@@ -64,6 +64,10 @@ var names = []string{"zaphod", "beeblebrox"}
 
 func TestMain(m *testing.M) {
 	upg, err := New(Options{})
+	if errors.Is(err, ErrNotSupported) {
+		fmt.Fprintln(os.Stderr, "Skipping tests, OS is not supported")
+		os.Exit(0)
+	}
 	if err != nil {
 		panic(err)
 	}