Snippets to install contain a wrong download url

Hey, the url in your snippets to install this awesome theme are wrong and point to a 404 location since https://raw.github.com/karel/chromegray/master/Custom.css doesn't exist.

I edited the URL to work with the current master version below: 

**Mac**
`curl -o ~/Library/Application\ Support/Google/Chrome/Default/User\ StyleSheets/Custom.css https://raw2.github.com/karelvuong/chromegray/master/Stable/Custom.css`

**PC (Windows Vista / 7 / 8)**
`powershell -Command "& {(new-object System.Net.WebClient).DownloadFile(""https://raw2.github.com/karelvuong/chromegray/master/Stable/Custom.css""","""$env:APPDATA\..\Local\Google\Chrome\User Data\Default\User StyleSheets\Custom.css""")}"`

**PC (Windows XP)**
`powershell -Command "& {(new-object System.Net.WebClient).DownloadFile(""https://raw2.github.com/karelvuong/chromegray/master/Stable/Custom.css""","""$env:APPDATA\Google\Chrome\User Data\Default\User StyleSheets\Custom.css""")}"`

**Fedora:**
`curl -o ~/.config/google-chrome/Default/User\ StyleSheets/Custom.css https://raw2.github.com/karelvuong/chromegray/master/Stable/Custom.css`