Issue
I see the error javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
Resolution
This indicates the Java keystore file on your dyno does not have the necessary certificates needed to trust a service you are invoking.
You can customize the keystore by creating a JDK overlay as described in Customizing the JDK on Dev Center.
Very often you can use the cacerts in your local JDK installation at $JAVA_HOME/jre/lib/security/cacerts or $JAVA_HOME/lib/security/cacerts, depending on your JDK version.