Fix the error "warning: deprecated Object#=~ ..."

[marcrohloff]

Ruby 2.7 now warns with

warning: deprecated Object#=~ is called on Launchy::Argv; it always returns nil

Cause: When Running Mint without any environment overrides set the browser_cmdline is Argv[xdg-open]

Alternative solutions:

• change if cmd =~ /%s/ then to if cmd.is_a?(String) && cmd =~ /%s/ then
• Add =~ and gsub! to Argv