Fixed usage of deprecated `insert_elem`

... since Elixir 0.11.0 it has been deprecated in favor of
`Tuple.insert_at`.
See https://github.com/elixir-lang/elixir/blob/v0.11.0/CHANGELOG.md
Used `test` over `think` to prevent Compile errors

... otherwise a compile error "function erlang_elem/2 undefined"
is thrown, or "about_tuples.exs:67: unquote called outside quote"
See #10 to help fixing this issue and being able to use `think`
Merge pull request #11 from real34/fix-tuples

Fixed tuples tests