override bootstrap legend width

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | na
| Fixed tickets | 
| Refs tickets  |
| License       | MIT
| Doc PR        | 

turns this: 
![shot_before](https://f.cloud.github.com/assets/350048/1274818/d3ca2190-2dcf-11e3-81d0-c634a010e755.png)

into this:
![shot_after](https://f.cloud.github.com/assets/350048/1274819/d7e6c97c-2dcf-11e3-8fb9-ce51076349b3.png)

look carefully at the top border of the `fieldset`