Skip certificate verification for sslmode "require"

This more appropriately aligns the behaviour of the library with
that advertised by the postgres documentation.

According to the table on the official documentation page
https://www.postgresql.org/docs/current/static/libpq-ssl.html,
the "require" mode should be used when:

"I want my data to be encrypted, and I accept the overhead. I trust that the network will make sure I always connect to the server I want."

This maps reasonably well to a TLS config that skips certificate verification.

conn.go

@@ -711,7 +711,9 @@ func configSSL(sslmode string, cc *ConnConfig) error {
 		cc.TLSConfig = &tls.Config{InsecureSkipVerify: true}
 		cc.UseFallbackTLS = true
 		cc.FallbackTLSConfig = nil
-	case "require", "verify-ca", "verify-full":
+	case "require":
+		cc.TLSConfig = &tls.Config{InsecureSkipVerify: true}
+	case "verify-ca", "verify-full":
 		cc.TLSConfig = &tls.Config{
 			ServerName: cc.Host,
 		}