I also ran into this issue on an RHEL 5 VM image where I am using openjdk 6.  It was the other note to look at TrustManager clued me in on a fix.  I tweak the invocation to add a trust setting for github; in my situation it resolves the peer authentication issue.
First grab the github certificate using openssl and keytool to make it accessible to java.
echo "" | openssl s_client -connect www.github.com:443 \
    -showcerts 2>/dev/null | openssl x509 -out github.cert
keytool -import -alias github \ 
    -file github.cert -storepass g8g8g8 \
    -keystore $HOME/g8.truststore
Now to rewrite the invocation with a script I call "G8":
g8 \
   \ -Djavax.net.ssl.trustStore=$HOME/g8.truststore \
   \ -Djavax.net.ssl.trustStorePassword=g8g8g8 \
   $*
Now try executing G8 -v typesafehub/akka-scala-sbt and I see things are much happier now.  I imagine setting a systemwide default truststore would may be better but I haven't figured that one out yet.