Use path.join to fix regression bug re. missing leading slashes

https://github.com/joliss/broccoli-sass/pull/28 introduced a regression bug that made output paths without leading slashes not work. No leading slash is the documented behavior in the README.

https://github.com/joliss/broccoli-sass/commit/10df2aa70871065ac0d6b0d500b7457ae9b53cf4#diff-168726dbe96b3ce427e7fedce31bb0bcL34
was changed to
https://github.com/joliss/broccoli-sass/commit/10df2aa70871065ac0d6b0d500b7457ae9b53cf4#diff-168726dbe96b3ce427e7fedce31bb0bcR35

I fixed it using `path.join`, which I think should always be used, except when called repeatedly for performance optimization.

I recommend releasing a new version asap as this will break the build for everyone who are not using leading slashes.