Change "no surge" icon to left-arrow

The ellipsis looked too much like a hyphen, which we use to signify no
data. This fits better with the up/down arrows.

https://www.pivotaltracker.com/story/show/83047320 [Delivers #83047320]
Merge pull request #48 from sealevelresearch/change-no-delta-icon

Change "no surge" icon to left-arrow