SourceMap: Fixes relative file path issue

[am11]

This fixes the issue described here: sass/node-sass#764.

When output_path is set as absolute path, the path should be resolved with respect to map file path for emitter.

[am11]

With this change, node-sass passes all 749 tests. (and without it, nine source-map related tests were failing)

@xzyfer, would it be possible to release libsass-v3.0.0-beta.2, so we can release node-sass v3.0.0-alpha.1 with all the awesome libsass new features? (-8

[coveralls]

Coverage remained the same at 80.72% when pulling 8f36c75 on am11:master into 4217540 on sass:master.

[xzyfer]

@am11 https://github.com/sass/libsass/releases/tag/3.2.0-beta.2

[am11]

@xzyfer, thank you! 👍 🎉

[am11]

Now, I would just need to attempt the code to compile against gcc 4.4.7 on o0ooold CentOS! 8-)