Changes

EdDSAEngine.initVerify(): Handle any non-EdDSAPublicKey X.509-encoded
pubkey
sun.security.x509.X509Key is a JDK-internal API, and should not be used 
directly. Instead of looking for an instance of that class, we check the 
primary encoding format of the PublicKey, and proceed if it is "X.509".

Travis CI: Use Trusty build environment
Travis CI recently bumped the default build environment from Trusty to 
Xenial. The latter does not enable installing Java 7 or 8, which we 
still test against.