fix broken coveralls reporting

[boneskull]

from recent builds:

> mocha@3.5.0 postcoverage /home/travis/build/[secure]/mocha
> istanbul-combine -d coverage -r lcov -r html coverage/reports/*/*.json
=============================== Coverage summary ===============================
Statements   : 100% ( 0/0 )
Branches     : 100% ( 0/0 )
Functions    : 100% ( 0/0 )
Lines        : 100% ( 0/0 )
================================================================================
[error] "2017-08-02T05:02:44.817Z"  'error from lcovParse: ' 'Failed to parse string'
[error] "2017-08-02T05:02:44.821Z"  'input: ' ''
[error] "2017-08-02T05:02:44.822Z"  'error from convertLcovToCoveralls'
/home/travis/build/[secure]/mocha/node_modules/coveralls/bin/coveralls.js:18
        throw err;
        ^
Failed to parse string