Remove sass-spec (sass#2873)

We only bought in sass-spec to catch regressions on the binding
layer that might cause crashes. As a result we've been happy to
leave our sass-spec version out of date because we weren't concerned
about correctness. With sass-spec migrating to hrx the ROI on
implementing hrx support.

package.json

@@ -80,7 +80,6 @@
     "object-merge": "^2.5.1",
     "read-yaml": "^1.0.0",
     "rimraf": "^2.5.2",
-    "sass-spec": "https://github.com/sass/sass-spec.git#dc2d573",
     "unique-temp-dir": "^1.0.0"
   }
 }

scripts/coverage.js

@@ -55,9 +55,8 @@ function coverage() {
             if (instrumentedfiles.length === sourcefiles.length) {
               fs.readdirSync('test').filter(function(file){
                 return false ||
-                  file.substr(-6)  === 'api.js' ||
-                  file.substr(-11) === 'runtime.js' ||
-                  file.substr(-7)  === 'spec.js';
+                  file.substr(-6) === 'api.js' ||
+                  file.substr(-11) === 'runtime.js';
               }).forEach(function(file){
                 mocha.addFile(
                   path.join('test', file)