The command line is /opt/libreoffice4.1/program/soffice.bin -headless -convert-to csv /path/to/your/file.xlsx
You'll need to update the path for libreoffice if you use a different version (the version is in the path above). You can put this into your Rails app by using
system("/opt/libreoffice4.1/program/soffice.bin --headless --convert-to csv /path/to/your/file.xlsx")
This will create a CSV file in the same directory as the original xlsx file.