Tests fail on newer SASS compilers

When running the test on SASS 3.2.10 all tests fail. I tried both on OSX and Linux.
The first 2 fail because the main compiled files was returned as a dependency twice, once with relative and once with absolute path. I'm not sure if in older compilers SASS would not include a comment line for it but now it does so returning `Seq(sassFile)` is no longer necessary.
The third test would fail because now the error message no longer fits. Now the message is more verbose so I changed the test assertion to use `contains` rather than `===`.
Also upped the sbt-plugin version. In the long run this should be thought as a cross-build so multiple versions are supported.