fix location of yasnippets of nsis-mode

[MichaelManert]

yas-current-dirs may be a list (as default) or a string - if set by user e.g. in init.el

update lib "cl" to "cl-lib"; "cl" is deprecated update variables "yas/xx" to "yas-xx"; "yas/xx" are deprecated

[mattfidler]

Thanks @MichaelManert