Update connection pool dependency

As indicated by the branch name, this pull request started as a simple update to the `connection_pool` dependency but then I continued to make a few additional changes. Everything is in atomic commits so feel free to take what you like and leave what you don't. A few of the changes (e.g. alphabetical gemspec) are purely a matter of taste.

I also added `Bundler::GemHelper.install_tasks` to the `Rakefile`. Again, it's a matter of taste, but it makes building and tagging releases easier and I couldn't think of any reason not to add it.

Once you merge in the `connection_pool` dependency update, please ship a new release of this gem. Thanks!