fix: added fallback mechanism to find right files

[EragonJ]

Reference: https://travis-ci.org/github/achingbrain/it/jobs/768135382

@achingbrain, as you can see, if you check the log from achingbrain/it closely, coveralls-lerna is not working as expected even in the latest build.

Error: ENOENT: no such file or directory, open 'index.js'
    at Object.openSync (fs.js:476:3)
    at Object.readFileSync (fs.js:377:35)
    at getSourceFromFile (/home/travis/build/achingbrain/it/node_modules/coveralls-lerna/lib/util/helpers.js:15:13)
    at /home/travis/build/achingbrain/it/node_modules/coveralls-lerna/lib/parse/parse-lcov.js:27:26
    at Array.forEach (<anonymous>)
    at convertLcovSectionToCoverallsSourceFile (/home/travis/build/achingbrain/it/node_modules/coveralls-lerna/lib/parse/parse-lcov.js:24:20)
    at /home/travis/build/achingbrain/it/node_modules/coveralls-lerna/lib/parse/parse-lcov.js:63:21
    at Array.map (<anonymous>)
    at Object.module.exports [as lcov] (/home/travis/build/achingbrain/it/node_modules/coveralls-lerna/lib/parse/parse-lcov.js:63:6)
    at parse (/home/travis/build/achingbrain/it/node_modules/coveralls-lerna/lib/parse.js:24:25) {
  errno: -2,
  syscall: 'open',
  code: 'ENOENT',
  path: 'in

I also encountered the same problem on my side and since the built lcov file is basically without workingDirectory information so that's why it doesn't work.

I am not sure whether coveralls-lerna did work well before, but somehow it's not working anymore because lcov.info is not with correct path information anymore.

Hope this quick fix gives you an idea about what the bug is happening now. I hope this change can be merged and the version can be released here, so feel free to advise this PR or just make a new one with proper solution.

Big thanks :)