Use 'expression' answer type for writing_expressions_1

Summary:
Using 'expression' rather than 'regex' seems to be more in-sync with the problem
statement.

Test Plan:
- Load up khan-exercises.
- Navigate to 'writing_expressions_1'.
- Solve a bunch of instances and verify that grading works correctly.

Reviewers: eater

Reviewed By: eater

Subscribers: peter, jack

Differential Revision: http://phabricator.khanacademy.org/D11941