convertLcovToCoveralls should convert absolute source paths to relative paths in output

Istanbul puts absolute file paths in the lcov output and node-coveralls passes them unchanged to Coveralls.
Coveralls cannot find the source file on GitHub because the path is wrong.
See https://coveralls.io/files/153108597 as an example.

This change converts absolute file paths from the lcov output to relative paths and everything works as expected again:
https://coveralls.io/files/153144314

The issue affects even this library's Coverall output: https://coveralls.io/files/143652117 :)