Fix dot transparency in IE8

I made some changes in slick.css to support  dot transparency in IE8

This is how IE8 currently displays Dots
![image](https://cloud.githubusercontent.com/assets/2680390/3078120/24159466-e45c-11e3-9509-f83b1ef8567b.png)

Here's how it looks after the changes
![image](https://cloud.githubusercontent.com/assets/2680390/3078123/695cf97e-e45c-11e3-9685-4ab57a78f4bb.png)
These screenshots were taken with IE8 on a Win7 VM from modern.ie

Demo: https://rawgit.com/salvador-guerrero/slick/master/index.html