Fixing Berkshelf Problem

This change fixes the following error which started happening with new versions of vagrant and virtual box:

```
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//*'
: No such file or 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

```