Introduce version number (#1255)

Changes to the instructions for interpreting tests will likely produce
new failures for consumers who are updating between revisions of
Test262. Introduce a machine-readable convention for signaling
substantive changes.

INTERPRETING.md

@@ -4,6 +4,11 @@ All tests are declared as text files located within this project's `test`
 directory. In order to execute Test262 tests, runtimes must observe the
 following semantics.
 
+**Note** When these instructions change in any substantive way, the `version`
+property of the JSON-formatted `package.json` file will be incremented. In this
+way, consumers who are transitioning between revisions of Test262 can more
+easily determine the cause of new test failures.
+
 ## Test Execution
 
 Test262 tests are only valid under the runtime environment conditions described

package.json

+{
+  "name": "test262",
+  "version": "1.0.0",
+  "description": "Test262 tests conformance to the continually maintained draft future ECMAScript standard found at http://tc39.github.io/ecma262/ , together with any Stage 3 or later TC39 proposals.",
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/tc39/test262.git"
+  },
+  "license": "BSD",
+  "bugs": {
+    "url": "https://github.com/tc39/test262/issues"
+  },
+  "private": true,
+  "homepage": "https://github.com/tc39/test262#readme"
+}