Accept more sequences for `codepost.roster.update()`

[josephlou5]

Example:

graders = codepost.roster.retrieve(course_id)
codepost.roster.update(course_id, superGraders=graders)

Above does not work because graders is of type codepost.models.abstract.linked_lists.APILinkedList. Only by explicitly creating a list object does it work:

graders = codepost.roster.retrieve(course_id)
codepost.roster.update(course_id, superGraders=list(graders))