Update selfsigned certificate #122

Previous certificate was not compatible with Java 11 / TLSv1.3. TLS Handshake
failed on "javax.net.ssl.SSLException: No preferred signature algorithm for
CertificateVerify". Creating a new certificate using a modern JDK / keytools
fixes the issue. Keystore and certificate have been created using following
command:

  keytool -genkey -keyalg RSA -alias selfsigned -keystore keystore.jks -storepass password  -keysize 2048