Bug 1196987: Update UUID returned to be a valid UUID 4; r=jgriffin

Currently the UUID returns the `{}` which is not valid and breaks
URI Templates used in WebDriver languages

testing/marionette/client/marionette/tests/unit/test_capabilities.py

@@ -86,3 +86,7 @@ def test_we_pass_in_required_capability_we_cant_fulfil_raises_exception(self):
         # Start a new session just to make sure we leave the browser in the
         # same state it was before it started the test
         self.marionette.start_session()
+
+    def test_we_get_valid_uuid_4_when_creating_a_session(self):
+        self.assertNotIn("{", self.marionette.session_id, 'Session ID has {} in it. %s ' \
+                         % self.marionette.session_id)

testing/marionette/driver.js

@@ -501,9 +501,10 @@ GeckoDriver.prototype.listeningPromise = function() {
 
 /** Create a new session. */
 GeckoDriver.prototype.newSession = function(cmd, resp) {
+  let uuid = uuidGen.generateUUID().toString();
   this.sessionId = cmd.parameters.sessionId ||
       cmd.parameters.session_id ||
-      uuidGen.generateUUID().toString();
+      uuid.substring(1, uuid.length - 1);
 
   this.newSessionCommandId = cmd.id;
   this.setSessionCapabilities(cmd.parameters.capabilities);