Error: Vagrant error using berkshelf

It appears to resolve if I install berkshelf seperately from the 'vagrant up' command. On most OS-X Systems that I've done this one I've had to use sudo also. It might be good to mention this in the README.md, but wanted to run this by you @hectcastro before I submitted a PR with the README.md fix. 

Here's a dump of the error.

$ vagrant up
Bringing machine 'riak1' up with 'virtualbox' provider...
Bringing machine 'riak2' up with 'virtualbox' provider...
Bringing machine 'riak3' up with 'virtualbox' provider...
[riak1] VirtualBox VM is already running.
[riak2] Importing base box 'opscode-ubuntu-12.04'...
[riak2] Matching MAC address for NAT networking...
[riak2] Setting the name of the VM...
[riak2] Clearing any previously set forwarded ports...
[riak2] Fixed port collision for 22 => 2222. Now on port 2200.
[riak2] Creating shared folders metadata...
[riak2] Clearing any previously set network interfaces...
[riak2] Preparing network interfaces based on configuration...
[riak2] Forwarding ports...
[riak2] -- 22 => 2200 (adapter 1)
[riak2] Running any VM customizations...
[riak2] Booting VM...
[riak2] Waiting for VM to boot. This can take a few minutes.
[riak2] VM booted and ready for use!
[riak2] Setting hostname...
[riak2] Configuring and enabling network interfaces...
[riak2] Mounting shared folders...
[riak2] -- /vagrant
[riak2] -- /tmp/vagrant-chef-1/lib
[riak2] -- /tmp/vagrant-chef-1/chef-solo-2/roles
[riak2] -- /tmp/vagrant-chef-1/chef-solo-1/cookbooks
[riak2] Running provisioner: shell...
[riak2] Running: inline script
stdin: is not a tty
dpkg-preconfigure: unable to re-open stdin: No such file or directory
Selecting previously unselected package liberror-perl.
(Reading database ...
61650 files and directories currently installed.)
Unpacking liberror-perl (from .../liberror-perl_0.17-1_all.deb) ...
Selecting previously unselected package git-man.
Unpacking git-man (from .../git-man_1%3a1.7.9.5-1_all.deb) ...
Selecting previously unselected package git.
Unpacking git (from .../git_1%3a1.7.9.5-1_amd64.deb) ...
Processing triggers for man-db ...
Setting up liberror-perl (0.17-1) ...
Setting up git-man (1:1.7.9.5-1) ...
Setting up git (1:1.7.9.5-1) ...
Installing berkshelf
Building native extensions.  This could take a while...
Building native extensions.  This could take a while...
ERROR:  Error installing berkshelf:
	ERROR: Failed to build gem native extension.

        /opt/chef/embedded/bin/ruby extconf.rb
checking for libxml/parser.h... no
-----
libxml2 is missing.  please visit http://nokogiri.org/tutorials/installing_nokogiri.html for help with installing dependencies.
-----
*** 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
	--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=/opt/chef/embedded/bin/ruby
	--with-zlib-dir
	--without-zlib-dir
	--with-zlib-include
	--without-zlib-include=${zlib-dir}/include
	--with-zlib-lib
	--without-zlib-lib=${zlib-dir}/lib
	--with-iconv-dir
	--without-iconv-dir
	--with-iconv-include
	--without-iconv-include=${iconv-dir}/include
	--with-iconv-lib
	--without-iconv-lib=${iconv-dir}/lib
	--with-xml2-dir
	--without-xml2-dir
	--with-xml2-include
	--without-xml2-include=${xml2-dir}/include
	--with-xml2-lib
	--without-xml2-lib=${xml2-dir}/lib
	--with-xslt-dir
	--without-xslt-dir
	--with-xslt-include
	--without-xslt-include=${xslt-dir}/include
	--with-xslt-lib
	--without-xslt-lib=${xslt-dir}/lib
	--with-libxslt-config
	--without-libxslt-config
	--with-pkg-config
	--without-pkg-config
	--with-libxml-2.0-config
	--without-libxml-2.0-config
	--with-pkg-config
	--without-pkg-config
	--with-libiconv-config
	--without-libiconv-config
	--with-pkg-config
	--without-pkg-config


Gem files will remain installed in /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/nokogiri-1.5.9 for inspection.
Results logged to /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/nokogiri-1.5.9/ext/nokogiri/gem_make.out
Berkshelf: Installing cookbooks
sudo
:
/opt/chef/embedded/bin/berks: command not found

mv:
cannot stat `/tmp/vagrant-chef-1/chef-solo-1/cookbooks/README.md/*'
: Not a directory

The following SSH command responded with a non-zero exit status.
Vagrant assumes that this means the command failed!

chmod +x /tmp/vagrant-shell && /tmp/vagrant-shell