.gemtest: remove the file

We use Travis CI nowadays.
Command::Ls: use brackets in the banner definition

Fix issue #855 (Installtion error in Mac OS X 10.8.2 with gem)
Command::Ls: use proper command API for the banner
Command::InstallCommand: honor ".gemrc" switches

Fix issue #666 (install-command doesn't seem to honor gemrc)

Also, prettify various messages from "install-command".
Command::InstallCommand: print explanation when a command is not found

Don't raise `NoMethodError`.