Composer updates

Thanks for a great piece of software!

Installing the module through composer would install it into the folder "silverstripe-languagprefix". With this change, it will now install into "languageprefix" automatically.

Relying on "dev-master" versions is a very bad practice. Explicitly stating what version of translatable that languageprefix is compatible with is allot more reliable.

Sidenote: It would be great if you could start tagging your commits for the same reasons as above. I do not find it very reliable having to specify "dev-master" in order to use this module.