Use string concat for lumbar paths. Fixes #7.

The generate task currently uses [path.join()](http://nodejs.org/api/path.html#path_path_join_path1_path2) to construct paths for lumbar.json. On Windows, `path.join()` uses backslashes instead of forward slashes, which is the opposite of what we want for lumbar (see \#7). 

We can avoid this by replacing path.join() with string concatenation, as the generate task isn't actually using any of the main benefits of path.join() (accepting `n` arguments, normalizing extra slashes). 

String concat doesn't look as nice, but will work better. Could also remove trailing slashes on paths if we need to, but it didn't seem necessary.