after installing bundler, you need to cd to msf3 folder and do bundle install
/home/opt/msf3 # 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 additional metadata from https://rubygems.org/.. Installing rake 10.1.0 Installing i18n 0.6.5 Installing multi_json 1.0.4 Installing activesupport 3.2.14 Installing builder 3.0.4 Installing activemodel 3.2.14 Installing arel 3.0.2 Installing tzinfo 0.3.37 Installing activerecord 3.2.14 Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension. /usr/bin/ruby1.8 extconf.rb extconf.rb:13:in `require': no such file to load -- mkmf (LoadError) from extconf.rb:13 Gem files will remain installed in /usr/lib/ruby/gems/1.8/gems/bcrypt-ruby-3.1.2 for inspection. Results logged to /usr/lib/ruby/gems/1.8/gems/bcrypt-ruby-3.1.2/ext/mri/gem_make.out An error occurred while installing bcrypt-ruby (3.1.2), and Bundler cannot continue. Make sure that `gem install bcrypt-ruby -v '3.1.2'` succeeds before bundling.
gem install bcrypt-ruby -v '3.1.2'
/usr/bin/ruby1.8 extconf.rb extconf.rb:13:in `require': no such file to load -- mkmf (LoadError) from extconf.rb:13