undefined "proposal" variable in DonorsChoose logic 

#### What's this PR do?
Previously, there was an invalid variable--`proposal`--in the logic which chose a proposal (out of the 10 proposals returned from the DonorsChoose API call.) 

We never caught this variable because our code rarely hit line 112. (It would only hit that point in the code if the first proposal in the list had a `costToComplete` property less than $10.) 

#### How should this be manually tested?
Running through the DonorsChoose donation flow using Postman.