Disable IPv6 for Postfix

@daveross came across an interesting bug with the default configuration -- since the `inet_protocols` defaults to "all", some network configurations will default to trying IPv6 and then giving up when an MX record won't resolve to it. (Try it yourself -- send to addresses at 10up.com vs. get10up.com). Disabling IPv6 and forcing IPv4 for all smtp connections seems to make this work correctly.
Add wildcard Nginx rules to load default VVV domains via xip.io

local.wordpress.*.xip.io
local.wordpress-trunk.*.xip.io
src.wordpress-develop.*.xip.io
build.wordpress-develop.*.xip.io
where * represents an IP address

The goal of these rules is to allow other devices on your network to
load these websites using URLs like the following
local.wordpress.192.168.1.10.xip.io
local.wordpress.10.0.1.20.xip.io

See #263
Install the mainline version of Nginx

In the world of Nginx, mainline can be considered the most feature
rich and the most stable. The "stable" release receives only bug
updates. A good explanation of this was posted by the nginx project
after the initial 1.7.x release from 1.6.x

http://nginx.com/blog/nginx-1-6-1-7-released/

I've used nginx 1.7.x in production since that day and have not had
any troubles. I think we should be good to move to mainline as our
default provisioning.
Merge pull request #462 from Varying-Vagrant-Vagrants/nginx-mainline

Install the mainline version of Nginx
Merge pull request #449 from salcode/nginx-xip-io-rules

Add wildcard Nginx rules to load default VVV domains via xip.io
Fixes typo in README

Fixes link to MIT License under Copyright/License section using malformed Markdown.
Update Composer packages on every provision

In #129, we moved from a composer.json to a global configuration
for packages such as phpunit. In the current state, changing the
version numbers for these packages only modifies new boxes.

Instead, we can check for and install Composer separately from
a self update and a global update. This allows version changes
to be applied with a standard `vagrant provision`.
global bin-dir is required for Composer packages
Upgrade PHPUnit to 4.3.x
Remove duplicate config of Composer bin-dir
Merge pull request #481 from chrishepner/master

Fix license link typo in README
Merge pull request #420 from zamoose/patch-3

Disable IPv6 for Postfix
Update phpMyAdmin to 4.2.11
Merge pull request #493 from Varying-Vagrant-Vagrants/phpadmin-update

[PR] Update phpMyAdmin to 4.2.11
Merge remote-tracking branch 'upstream/composer-provision'