config.ru: setup application environment with Bundler

Commit "manage load path with Bundler" introduced Bundler as a runtime
dependency so that SequenceServer doesn't have to worry about managing
`LOAD_PATH` itself.  However, until now SequenceServer leveraged Bundler
at only one of the possible 'entry points' to the application:
`bin/sequenceserver` (for running SequenceServer standalone).

This commit employs Bundler to setup the application environment in `config.ru`
which is the entry point for Rack adapters.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>

config.ru

@@ -1,5 +1,6 @@
-# ensure 'lib/' is in the load path
-require File.join(File.dirname(__FILE__), 'lib', 'sequenceserver')
+require 'rubygems'
+require 'bundler/setup'
+require 'sequenceserver'
 
 SequenceServer::App.init
 run SequenceServer::App