Update the Linux install instructions (and fix a build issue)

This addresses at least some of the issues discussed in #105 

Also, before this fix, `rp-build-app` failed with:

    + cp -f ruby_help.tmpl ruby_help.h
    + ../../server/bin/qt-doc.rb -o ruby_help.h
    /git/sonic-pi/app/server/sonicpi/lib/sonicpi/docsystem.rb:73:in `block in docs_html_map': undefined me
        from /git/sonic-pi/app/server/sonicpi/lib/sonicpi/docsystem.rb:52:in `each'
        from /git/sonic-pi/app/server/sonicpi/lib/sonicpi/docsystem.rb:52:in `docs_html_map'
        from ../../server/bin/qt-doc.rb:164:in `<main>'