execute "install_gems" as root to fix build error

execute "install_gems" as root to fix build error

Using fast_stack (0.1.0) 
Using fast_xor (1.1.2) from https://github.com/CodeMonkeySteve/fast_xor.git (at master) 
Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.

    /opt/rbenv/versions/2.0.0-p247/bin/ruby extconf.rb 
creating Makefile
/opt/rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/mkmf.rb:2117:in `initialize': Permission denied - Makefile (Errno::EACCES)
        from /opt/rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/mkmf.rb:2117:in `open'
        from /opt/rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/mkmf.rb:2117:in `create_makefile'
        from extconf.rb:2:in `<main>'


Gem files will remain installed in /opt/discourse/vendor/bundle/ruby/2.0.0/bundler/gems/fast_xor-85b79ec6d116 for inspection.
Results logged to /opt/discourse/vendor/bundle/ruby/2.0.0/bundler/gems/fast_xor-85b79ec6d116/ext/xor/gem_make.out
An error occurred while installing fast_xor (1.1.2), and Bundler cannot continue.
Make sure that `gem install fast_xor -v '1.1.2'` succeeds before bundling.