Use consistent styling for number of commits in CI nav sidebar

Before:

![Screen_Shot_2015-09-21_at_15.00.28](https://gitlab.com/gitlab-org/gitlab-ce/uploads/9005f606842936d415a3b67890b8af88/Screen_Shot_2015-09-21_at_15.00.28.png)


After:

![Screen_Shot_2015-09-21_at_15.00.43](https://gitlab.com/gitlab-org/gitlab-ce/uploads/08c897b65f82fab5f2c386a0d21c64af/Screen_Shot_2015-09-21_at_15.00.43.png)


See merge request !1375