Small bugfixes for "script/generate leaf <leaf>"

Fixes some bugs when trying to generate a new leaf:

libs/generator.rb:61:in `leaf': uninitialized constant Autumn::Generator::Pathname (NameError)
libs/coder.rb:158:in kind_of?: class or module required (TypeError)
No such file or directory - leaves/fortune/views/about.txt.erb (Errno::ENOENT)