Add kerberos package to ubuntu installation instructions

Stack trace of error from bundle install

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    /home/dzaporozhets/.rvm/rubies/ruby-2.1.5/bin/ruby -r ./siteconf20150116-17237-rl0g9.rb extconf.rb 
checking for main() in -lc... yes
checking for krb5_init_context() in -lkrb5... no
creating Makefile

make "DESTDIR=" clean

make "DESTDIR="
compiling ruby_krb5_auth.c
ruby_krb5_auth.c:24:18: fatal error: krb5.h: No such file or directory
 #include <krb5.h>
compilation terminated.
make: *** [ruby_krb5_auth.o] Error 1

make failed, exit code 2

Gem files will remain installed in /home/dzaporozhets/.rvm/gems/ruby-2.1.5/gems/timfel-krb5-auth-0.8 for inspection.
Results logged to /home/dzaporozhets/.rvm/gems/ruby-2.1.5/extensions/x86_64-linux/2.1.0/timfel-krb5-auth-0.8/gem_make.out
An error occurred while installing timfel-krb5-auth (0.8), and Bundler cannot continue.
Make sure that `gem install timfel-krb5-auth -v '0.8'` succeeds before bundling.