rbenv ree install fails

On my Ubuntu 12.04 64 big install:
# uname -a
Linux rhapunder 3.2.0-23-generic #36-Ubuntu SMP Tue Apr 10 20:39:51 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux

with rbenv:
# rbenv
rbenv 0.3.0
usage: rbenv <command> [<args>]
...
---snip---
I am unable to install any ree-1.8.7-* version because of the requirement for libreadline5-dev:
# rbenv install ree-1.8.7-2011.12
Downloading http://rubyenterpriseedition.googlecode.com/files/ruby-enterprise-1.8.7-2011.12.tar.gz...
Installing ruby-enterprise-1.8.7-2011.12...

BUILD FAILED

Inspect or clean up the working tree at /tmp/ruby-build.20120514150633.10784
Results logged to /tmp/ruby-build.20120514150633.10784.log

Last 10 log lines:
Some required software is not installed.
But don't worry, this installer will tell you how to install them.
Press Enter to continue, or Ctrl-C to abort.
--------------------------------------------
Installation instructions for required software

 * To install GNU Readline development headers:
   Please run apt-get install libreadline5-dev as root.
---snip---
When I try to install libreadline5-dev it refers me to something else, and then the something else fails too:


# apt-get install libreadline5-dev
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package libreadline5-dev is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  libreadline-gplv2-dev:i386 lib64readline-gplv2-dev:i386 libreadline-gplv2-dev

E: Package 'libreadline5-dev' has no installation candidate
---snip---
The first two of the above fail to install for me, and the third causes  the original target rbenv install to fail.