Add "maintain_test_schema!" to spec_helper.rb

Rails 4.1 introduces Migration.maintain_test_schema!, deprecating the
old "db:test:prepare" rake task.
Prevent ConverterApi#convert_to returning Infinity

If the conversion involves a divisor which is zero, the method would
return Infinity. This is both incorrect (the return should also be
zero), but cannot be encoded into JSON, resulting in a 500 Server Error.

If the conversion results in Infinity, we return zero instead.

Closes quintel/etsource#820.