how do I fix this problem ?
andika@andika-HP:~/andikanug$ rails s Warning: Running `gem pristine --all` to regenerate your installed gemspecs (and deleting then reinstalling your bundle if you use bundle --path) will improve the startup performance of Spring. /var/lib/gems/2.1.0/gems/execjs-2.5.2/lib/execjs/runtimes.rb:48:in `autodetect': Could not find a JavaScript runtime. See GitHub - rails/execjs: Run JavaScript code from Ruby for a list of available runtimes. (ExecJS::RuntimeUnavailable) from /var/lib/gems/2.1.0/gems/execjs-2.5.2/lib/execjs.rb:5:in `<module:ExecJS>' from /var/lib/gems/2.1.0/gems/execjs-2.5.2/lib/execjs.rb:4:in `<top (required)>' from /var/lib/gems/2.1.0/gems/uglifier-2.7.1/lib/uglifier.rb:3:in `require' from /var/lib/gems/2.1.0/gems/uglifier-2.7.1/lib/uglifier.rb:3:in `<top (required)>' from /var/lib/gems/2.1.0/gems/bundler-1.9.4/lib/bundler/runtime.rb:76:in `require' from /var/lib/gems/2.1.0/gems/bundler-1.9.4/lib/bundler/runtime.rb:76:in `block (2 levels) in require' from /var/lib/gems/2.1.0/gems/bundler-1.9.4/lib/bundler/runtime.rb:72:in `each' from /var/lib/gems/2.1.0/gems/bundler-1.9.4/lib/bundler/runtime.rb:72:in `block in require' from /var/lib/gems/2.1.0/gems/bundler-1.9.4/lib/bundler/runtime.rb:61:in `each' from /var/lib/gems/2.1.0/gems/bundler-1.9.4/lib/bundler/runtime.rb:61:in `require' from /var/lib/gems/2.1.0/gems/bundler-1.9.4/lib/bundler.rb:134:in `require' from /home/andika/andikanug/config/application.rb:7:in `<top (required)>' from /var/lib/gems/2.1.0/gems/railties-4.2.1/lib/rails/commands/commands_tasks.rb:78:in `require' from /var/lib/gems/2.1.0/gems/railties-4.2.1/lib/rails/commands/commands_tasks.rb:78:in `block in server' from /var/lib/gems/2.1.0/gems/railties-4.2.1/lib/rails/commands/commands_tasks.rb:75:in `tap' from /var/lib/gems/2.1.0/gems/railties-4.2.1/lib/rails/commands/commands_tasks.rb:75:in `server' from /var/lib/gems/2.1.0/gems/railties-4.2.1/lib/rails/commands/commands_tasks.rb:39:in `run_command!' from /var/lib/gems/2.1.0/gems/railties-4.2.1/lib/rails/commands.rb:17:in `<top (required)>' from /home/andika/andikanug/bin/rails:8:in `require' from /home/andika/andikanug/bin/rails:8:in `<top (required)>' from /var/lib/gems/2.1.0/gems/spring-1.3.4/lib/spring/client/rails.rb:28:in `load' from /var/lib/gems/2.1.0/gems/spring-1.3.4/lib/spring/client/rails.rb:28:in `call' from /var/lib/gems/2.1.0/gems/spring-1.3.4/lib/spring/client/command.rb:7:in `call' from /var/lib/gems/2.1.0/gems/spring-1.3.4/lib/spring/client.rb:26:in `run' from /var/lib/gems/2.1.0/gems/spring-1.3.4/bin/spring:48:in `<top (required)>' from /var/lib/gems/2.1.0/gems/spring-1.3.4/lib/spring/binstub.rb:11:in `load' from /var/lib/gems/2.1.0/gems/spring-1.3.4/lib/spring/binstub.rb:11:in `<top (required)>' from /usr/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require' from /usr/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:55:in `require' from /home/andika/andikanug/bin/spring:13:in `<top (required)>' from bin/rails:3:in `load' from bin/rails:3:in `<main>' andika@andika-HP:~/andikanug$