fix size of breadcrumb separator

cc @icewind1991 @owncloud/designers @PVince81 

Before
![breadcrumb-before](https://cloud.githubusercontent.com/assets/245432/3483629/7f019140-0392-11e4-8800-fbc55c6108bf.png)
After
![breadcrumb-after](https://cloud.githubusercontent.com/assets/245432/3483630/83784c1e-0392-11e4-93f1-5667a8f2a0a5.png)