[MEDIUM] SS-2018-005: isDev and isTest unguarded

PKSA-tdy9-8my8-6s7g GHSA-r32j-mr8p-hfp8

Affected package: silverstripe/framework

Affected version: >=4.0.0,<4.0.4|>=4.1.0,<4.1.1

Reported by:
GitHub, FriendsOfPHP/security-advisories