Fix 'babushka edit' command, when EDITOR/BABUSHKA_EDITOR/VISUAL environment variable not set

Without this fix:

$ babushka edit npm
/usr/local/babushka/lib/babushka/cmdline.rb:106: undefined method `which' for Babushka::Cmdline:Class (NoMethodError)
	from /usr/local/babushka/lib/babushka/source_pool.rb:68:in `call'
	from /usr/local/babushka/lib/babushka/source_pool.rb:68:in `find_or_suggest'
	from /usr/local/babushka/lib/babushka/cmdline.rb:101
	from /usr/local/babushka/lib/babushka/cmdline/parser.rb:28:in `call'
	from /usr/local/babushka/lib/babushka/cmdline/parser.rb:28:in `run'
	from /usr/local/babushka/lib/babushka/base.rb:60:in `run'
	from /usr/local/bin/babushka:25