Unable to gem install couchbase

[root@cen-1908 ~]# ruby -v
ruby 1.8.7 (2011-06-30 patchlevel 352) [x86_64-linux]
[root@cen-1908 ~]# gem --version
1.8.10

Followed the instructions listed here:-
https://github.com/couchbase/couchbase-ruby-client

[root@cen-1908 ~]# gem install couchbase
Building native extensions. This could take a while...
......
ERROR: Error installing couchbase:
ERROR: Failed to build gem native extension.

/usr/bin/ruby extconf.rb

• • • extconf.rb failed ***
      Could not create Makefile due to some reason, probably lack of
      necessary libraries and/or headers. Check the mkmf.log file for more
      details. You may need configuration options.

Provided configuration options:
--with-opt-dir
--without-opt-dir
--with-opt-include
--without-opt-include=${opt-dir}/include
--with-opt-lib
--without-opt-lib=${opt-dir}/lib
--with-make-prog
--without-make-prog
--srcdir=.
--curdir
--ruby=/usr/bin/ruby
--with-libcouchbase-dir
--without-libcouchbase-dir
--with-libcouchbase-include
--without-libcouchbase-include=${libcouchbase-dir}/include
--with-libcouchbase-lib
--without-libcouchbase-lib=${libcouchbase-dir}/lib
extconf.rb:86: undefined method `<<' for nil:NilClass (NoMethodError)

Gem files will remain installed in /usr/lib/ruby/gems/1.8/gems/couchbase-1.1.4 for inspection.
Results logged to /usr/lib/ruby/gems/1.8/gems/couchbase-1.1.4/ext/couchbase_ext/gem_make.out