Update Elixir Sieve to conform to the README

Update the solution template to remove the private method stub
Update the example to use the Sieve of Eratosthenes

The README expects the implementation to use the Sieve of Eratosthenes,
but the previous solution template encouraged a different algorithm by
providing a stub for private method with a particular interface.
Merge pull request #1349 from drueck/update-elixir-sieve

Update Elixir Sieve to conform to the README