Refactor how data is imported via CSV

Based on feedback from @desmondmorris : https://github.com/DoSomething/mbp-user-import/pull/2 the way data is processed from CSV import files results in code that is dependent on the file header rather than hard coded idex positions. This is a refactoring to a more elegant and flexible solution.
This line seems redundant given what you're doing on the next line.