Bump the minimum requirements to WordPress 3.7.1
Remove back-compat time constants

These were added to core in 3.5, since we bumped the minimum requirements to 3.7.1 we can get rid of our back-compat versions.
Stop checking the minimum supported PHP version

WordPress 3.7.1 has a minimum requirement of PHP 5.2.4, no need for us to also check it.
Get rid of some stray double line-breaks
Stop testing against 3.6

Our new minimum requirement is 3.7.1
Bump the readme to require WordPress 3.7.1
Merge pull request #442 from humanmade/minimum-3-7

Bump minimum required WordPress version to 3.7.1