root@kali:/opt/metasploit-framework# bundle install
Don't run Bundler as root. Bundler can ask for sudo if it is needed, and
installing your bundle as root will break this application for all non-root
users on this machine.
Fetching gem metadata from https://rubygems.org/.........
Fetching version metadata from https://rubygems.org/..
Resolving dependencies......
Using rake 10.4.2
Using i18n 0.7.0
Using minitest 4.7.5
Using multi_json 1.11.1
Using thread_safe 0.3.5
Using tzinfo 0.3.44
Using activesupport 4.0.13
Using builder 3.1.4
Using erubis 2.7.0
Using rack 1.5.5
Using rack-test 0.6.3
Using actionpack 4.0.13
Using mime-types 2.6.1
Using mail 2.6.3
Using actionmailer 4.0.13
Using activemodel 4.0.13
Using activerecord-deprecated_finders 1.0.4
Using arel 4.0.2
Using activerecord 4.0.13
Using arel-helpers 2.1.0
Installing ffi 1.9.8 with native extensions

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

/usr/bin/ruby2.1 extconf.rb

mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h

extconf failed, exit code 1

Gem files will remain installed in /var/lib/gems/2.1.0/gems/ffi-1.9.8 for inspection.
Results logged to /var/lib/gems/2.1.0/extensions/x86-linux/2.1.0/ffi-1.9.8/gem_make.out
An error occurred while installing ffi (1.9.8), and Bundler cannot continue.
Make sure that gem install ffi -v '1.9.8' succeeds before bundling.