Look for Bob first, not TeenagerTest::Bob

The first error after running the Very Friendly Intro said:

test_statement_containing_question_mark(TeenagerTest):
NameError: uninitialized constant TeenagerTest::Bob
    bob_test.rb:10:in `setup'

This fixes the error message so students are expected to
declare Bob as a top-level constant.
Merge pull request #450 from markijbema/check-for-bob

Look for Bob first, not TeenagerTest::Bob