Fix customProbability in JSON response

Created by: ehunter-usgs

customProbability was being returned as a string in the JSON feed,

• http://localhost:8000/ws/designmaps/asce41-17.json?latitude=34&longitude=-118&siteClass=C&customProbability=0.1
• http://localhost:8000/ws/designmaps/asce41-13.json?latitude=34&longitude=-118&siteClass=C&customProbability=0.1