Reducing opacity of census tract choropleth layer

Don't hate me, @cmc333333. I thought I was in Dreamweaver.

(But seriously, if doing this is overstepping my bounds, distracting, or not useful, just slap my hand and let me know!)

#### To resolve [FLM-211](https://jira.demo.cfpb.gov/browse/FLM-211)
Helps punch-up the visibility of the scaled circles, particularly for the middle-range tones on the color ramp. Tract layer should have just enough visibility of the overall minority distribution.

![flm-211_tract-opacity_02](https://cloud.githubusercontent.com/assets/1689222/3984671/fcdc32c0-2889-11e4-9ff6-f591ba4091ec.gif)