Install bundler Gem tasks in bundler namespace

* The release task in Bundler >= 1.3.1 depends on the build task, which
  conflicts with our build task, resulting in a webkit_server build
  instead of a gem build.

[ci skip]

Rakefile

@@ -3,7 +3,9 @@ require 'rspec/core/rake_task'
 require_relative './lib/capybara_webkit_builder'
 require 'appraisal'
 
-Bundler::GemHelper.install_tasks
+namespace :bundler do
+  Bundler::GemHelper.install_tasks
+end
 
 desc "Generate a Makefile using qmake"
 file 'Makefile' do