Add a 'version.txt' file to the tarball

This will enable the vector build to know what it got

.gitignore

@@ -1,5 +1,6 @@
 /node_modules
 /lib
 
-# tarball created by 'npm pack'
+# version file and tarball created by 'npm pack'
+/version.txt
 /matrix-react-sdk-*.tgz

package.json

@@ -17,7 +17,7 @@
     "build": "babel src -d lib --source-maps",
     "start": "babel src -w -d lib --source-maps",
     "clean": "rimraf lib",
-    "prepublish": "npm run build"
+    "prepublish": "npm run build; git rev-parse HEAD > version.txt"
   },
   "dependencies": {
     "classnames": "^2.1.2",