fix capa textline to properly use inline if specified when using math preprocessor

When using the dynamic math javascript preprocessor feature, textline does not honor the "inline" flag, because of a div which is fixed to be `display:block`. 

This PR fixes that bug, using the same approach as previously employed (https://github.com/edx/edx-platform/pull/845).

1 line, 12 character fix.

@chrisndodge ?