Bump required version, if needed

[hadley]

Extracted from #484

use_dependency("rlang", "Imports", ">= 0.1.2") should only change the version number if it's lower than 0.1.2 (or not specified)

[hadley]

This will require changing the version argument to use_dependency() to min_version, but use_dependency() is not exported, and we only use >= restriction in existing code. I think this change will also simplify the implementation.