event-last-push styling fix

When viewing a fresh install of GitLab on a mobile device after a branch with a name of sufficient length, the broken styling can be seen as follows:

![Screen Shot 2015-01-05 at 9.37.32 PM](https://gitlab.com/uploads/cloudwindhk/gitlab-ce/9b3ca84f1a/Screen_Shot_2015-01-05_at_9.37.32_PM.png)

I modified the css for .event-last-push and .event-last-push-text to make it look correct:

![Screen Shot 2015-01-05 at 9.37.50 PM](https://gitlab.com/uploads/cloudwindhk/gitlab-ce/43d5a912b3/Screen_Shot_2015-01-05_at_9.37.50_PM.png)

As this is a purely css change, I don't believe it will break any tests. The only occurrence of .event-last-push* is in ./app/views/events/_event_last_push.html.haml

See merge request !279