Replace upgrade notification close x with &times

The multiplication sign usually creates a better looking close button than the letter x, and it's been bugging me whenever vimium updates.
 
Before (x):
![image](https://cloud.githubusercontent.com/assets/1953083/3736329/aba2c888-172c-11e4-9f66-23b52dbc956d.png)

After (&times;):
![image](https://cloud.githubusercontent.com/assets/1953083/3736334/c02047b8-172c-11e4-9c5a-c97e1b136098.png)