e2e: one seleniuum session at a time

We are less likely to get whacky timeouts from protractor/Sauce
interaction if we run only one selenium session at a time.

Author: laurelnaiad

browser/dev-tasks/e2e/protractor.js

@@ -15,6 +15,7 @@ var ptorConfig = {
   rootElement: 'html',
   chromeOnly: false,
   framework: 'cucumber',
+  maxSessions: 1,
   specs: require('globule').find(
     path.resolve(ctx.paths.projectRoot, 'specs/features/**.feature')
   ),