metadata: require puppetlabs-stdlib 4.2.0 and up

Required as of c87744aab94a8fae81669dbffb1d44a48a3f3da1 (#509). Only 4.2.0 and up treats integers in strings as being an integer.

Should push a new version to Forge since anyone on less than 4.2.0 will likely be having trouble.

Fixes #537