Better WordPress Coding standards.

This minor commit kicks off code improvements to _s so that it’s fully in line with WordPress Coding Standards rules.
Better build config for Travis CI. [skip ci]
Better rules for Travis CI.
The commit https://github.com/WordPress-Coding-Standards/WordPress-Coding-Standards/commit/6986e21182def2111135e46493484581ac494632#diff-9dbd3b63f5d2589b5eb1701e92893803 began (in part) breaking Travis CI builds in _s. There’s most certainly room for improvement to _s code with regard to WordPress Coding Standards, but let’s exclude WordPress.Variables.GlobalVariables and correct all coding standards errors on localhost before removing the excludes from our GitHub-hosted ruleset.
Merge pull request #1 from Automattic/master

Minor changes & new travis rules