Update sunrise and allow newer versions of WordPress to work

[jeremyfelt]

Addresses #303:

• Update how a network is defined by assuming only one network exists and is defined in the configuration.
• Watch for the likely not to happen scenario where a second network is installed.
• Catch a case where the site_admins network option may not be properly set during installation locally.
• Allow downloads.wordpress.org URLs to survive the protocol relative filter so that local updates will work nicely.

This takes a slightly different approach from @Ipstenu in #378 as there are quite a few gotchas hanging out there when running this outside of the Vagrant environment and/or when installing and upgrading.

There's probably room for future improvement, especially once the Vagrant box has been updated to use a newer version of WordPress by default. We could then rewrite a bit more and make a few more assumptions about the environment. This should work as a pretty good stop-gap in the meantime.

I've tested:

• New install of the environment in WordPress 3.8.1
• Upgrade from WordPress 3.8.1 to 4.3.1
• New install of the environment in WordPress 4.3

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[gnarf]

Just tested, everything looks great - Thanks so much!