you need the ruby bindings to it. From Synaptic (easier for me) I looked up all packages with ruby in their name...installed some that I thought I needed, then it worked.
Can't tell you which package fixed this because I installed quite a few packages.

msfconsole
<snip>logo</snip>
=[ msf v3.0-beta-dev
+ -- --=[ 116 exploits - 99 payloads
+ -- --=[ 17 encoders - 4 nops
=[ 17 aux

msf >