Make yammer plugin work on newer Jenkins versions

Hi Matthew,
I need to run your Yammer plugin on Jenkins 1.505 which didn't work because I got this exception: 

    Sending Yammer notification...
    ERROR: org.bouncycastle.asn1.ASN1InputStream.readObject()Lorg/bouncycastle/asn1/DERObject; (Java::JavaLang::NoSuchMethodError)
    org.jruby.ext.openssl.x509store.PEMInputOutput.readAuxCertificate(PEMInputOutput.java:1120)

I tried upgrading the dependency on jruby-openssl which Worked For Me[tm].

Thank you for building this plugin, 
best regards, Jörg