Stop using fs *Sync methods

[XhmikosR]

Minus fs.existsSync.

[jtwebman]

This is a command-line tool that is run in pipelines?

[jtwebman]

Since this library doesn't seem to be supported anymore I fix a bunch of things on a fork if you want to check it out and are still pulling the library into your packages: https://github.com/jtwebman/coveralls-next But it is a command-line tool not a service with multiple requests so there is no harm in using sync calls.