Avoid inconsistently large checkbox margin

Avoid inconsistently large margin on first-child checkbox which is followed by another checkbox.

Re. #2033

Results in Firefox:

![screenshot - 140414 - 15 19 00](https://cloud.githubusercontent.com/assets/192853/2695612/934f40c4-c3d8-11e3-9f5b-1aa8e7e5a27e.png)

![screenshot - 140414 - 15 21 11](https://cloud.githubusercontent.com/assets/192853/2695613/9cb36758-c3d8-11e3-98c3-d7bcf09b844f.png)

![screenshot - 140414 - 15 21 31](https://cloud.githubusercontent.com/assets/192853/2695615/9f66abe0-c3d8-11e3-9c3b-dc9fb8531be6.png)

![screenshot - 140414 - 15 21 59](https://cloud.githubusercontent.com/assets/192853/2695617/a19c4cbc-c3d8-11e3-8b07-513851d7d6f6.png)