chore: set version to 3.7.0-alpha (#240)

While there, make sure we don't always skip a currently failing
riseupvpn test, and slightly clarify the readme.

internal/engine/experiment/riseupvpn/riseupvpn_test.go

@@ -278,7 +278,9 @@ func TestFailureGeoIpServiceBlocked(t *testing.T) {
 }
 
 func TestFailureGateway(t *testing.T) {
-	t.Skip("test currently not WAI - will restore after release")
+	if testing.Short() {
+		t.Skip("skip test in short mode")
+	}
 	var testCases = [...]string{"openvpn", "obfs4"}
 	eipService, err := fetchEipService()
 	if err != nil {