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.