Raise exception if <file_name> isn't passed to bin/ansible-vault

Ran into this on my first run through with vault.  I was asked to enter my password and got:

    ansible-vault create --debug
    Vault password:
    Confirm Vault password:
    Traceback (most recent call last):
      File "<virtual_env>/bin/ansible-vault", line 211, in main
        fn(args, options, parser)
      File "<virtual_env>bin/ansible-vault", line 126, in execute_create
        this_editor = VaultEditor(cipher, password, args[0])
    IndexError: list index out of range

    ERROR: list index out of range

This pull request checks that we at least have the one required argument before we continue.