convertLcovToCoveralls should output paths with slashes even on windows.

When used on windows, the library sends file paths that use backslashes as separators. This patch simply replaces the system separator with slashes.

[Without the patch](https://coveralls.io/builds/797345), it's [impossible to inspect files](https://coveralls.io/files/204861636). [With the patch](https://coveralls.io/builds/797349), [it works](https://coveralls.io/files/204862687).