Updating jquery.flot.d.ts. Fixing type for seriesOptions.color

Flot specifies that the color is either a number or a string. 

See: https://github.com/flot/flot/blob/master/API.md

------------
> The format of a single series object is as follows:
```
{
      color: color or number
      ...
}
```

> If you don't specify color, the series will get a color from the 
> auto-generated colors. The color is either a CSS color 
> specification (like "rgb(255, 100, 123)") or an integer that 
> specifies which of auto-generated colors to select, e.g. 0 will get 
> color no. 0, etc.