Improve srcFile suffix replacement

When using gulp.src, if you set a base dir then you can end up with relative paths that start with a `../`.

The existing regex for replacing the file extension doesn't work in this case, even though the * is non-greedy.

We also perhaps need to consider what a file extension is. Is it the last part of the filename, delimited by the `.` or is it everything from the first `.` in the filename (not dirname) part of the path?

I've assumed the later in this suggested change to the regex. It matches `.` followed by anything not another `.` or a `/`, and then any other characters to the end of the string. Or just `.` at the end of the string.
How about this?

```js
var relative = srcFile.relative;
var destFileRelative = self._ext ?
    relative.substr(0, relative.length - path.extname(relative)) + self._ext :
    relative;
```