Update lower-left job details content with consistent linespace

This issue was referenced in PR https://github.com/mozilla/treeherder-ui/pull/125.

Basically we have a strange upward shift of the contents in the lower left job panel. It appears to be driven by a default bootstrap css value of `margin-bottm: 5px` for all labels [here](https://github.com/mozilla/treeherder-ui/blob/master/webapp/app/css/bootstrap.css#L1645).

I assume we want to just override that value on this panel element in treeherder.css, so this is that change. We apply a top and bottom margin instead. I've included a before and after you can quickly flip between them and see the difference. It's subtle, but much better.

![leftjobpanel_current](https://cloud.githubusercontent.com/assets/3660661/3894363/7cb95a2e-2243-11e4-8721-bc06806a4b9e.jpg)
![leftjobpanel_proposed](https://cloud.githubusercontent.com/assets/3660661/3894364/80b31a52-2243-11e4-81cb-299f911c25e3.jpg)

It also takes up less net space for given content, since we are consuming 4px of margin rather than 5.

Tested on Windows:
FF Release **31.0**
Chrome Latest Release **36.0.1985.125 m**

Adding @camd for visibility.