Update API endpoint in Student Enrollment Trends

[jaronheard]

Per a comment from @arailsya in #197, we need to update the API endpoint in
packages/2018-neighborhood-development/src/state/student-enrollment-trends/actions.js

However, in order to do this, we need hackoregon/neighborhoods-2018#91 to be resolved.

@jaronheard I think you are hitting the wrong API endpoint, @hassanshamim made a new endpoint that filtered the data for the visualization: http://service.civicpdx.org/neighborhood-development/api/school_demographics_racial_counts