hello world: make topics match the content

[azul]

This used to be println which actually occures nowhere in the excercise.
Change this to:

• strings - because the function returns a string and we compare strings
• test_driven_development - as this is the first excercise and people learn the red, green flow.

Both topics are taken from https://github.com/exercism/problem-specifications/blob/master/TOPICS.txt

This also addresses #529 for hello world.

[petertseng]

I am confused as to why println was ever a topic for hello-world. I cannot remember any point in the history of the exercise where println! was used. At the very most, back in the two-fer-equivalent version of hello-world, there would have been format!, but never println. Shrug. The new topics seem much better.

Other maintainers have about 24 hours to object.