Support boolean values in $_ENV and $_SERVER (phpdotenv#457)

Merge branch '5.0'

Apply fixes from StyleCI (phpdotenv#451)

Fixed indentation

Update CONTRIBUTING.md

Tweaks

Replaced whitelists by allow lists

Improved static analysis

Merge branch '2.6' into 3.6

Revert "Update .travis.yml"

This reverts commit f52b0a0.