Update curl install script

For me, the curl command doesn't get the redirect page's script

The install script is [here](https://raw.githubusercontent.com/robbyrussell/oh-my-zsh/master/tools/install.sh),
but the current curl command only returns 
```
<a href="https://raw.github.com/robbyrussell/oh-my-zsh/master/tools/install.sh">Moved Permanently</a>.
```
for me. This edit works out though.