fix source file path generation

When running ```coveralls``` on libtomcrypt I got the following error:

```
Traceback (most recent call last):
  File "/usr/local/bin/coveralls", line 9, in <module>
    load_entry_point('cpp-coveralls==0.2.1', 'console_scripts', 'coveralls')()
  File "/usr/local/lib/python2.7/dist-packages/cpp_coveralls/__init__.py", line 69, in run
    cov_report = coverage.collect(args)
  File "/usr/local/lib/python2.7/dist-packages/cpp_coveralls/coverage.py", line 280, in collect
    with io.open(source_file_path, encoding=args.encoding) as src_file:
IOError: [Errno 2] No such file or directory: '<my working path>/libtomcrypt/src/math/src/math/multi.c'
```

After some debugging I came up with the proposed patch