Require rb-fsevent when needed, in _configure method

[jirutka]

This fixes an error I’ve introduced in #417, sorry for that.

The problem is: when the OS version is >= 13, then it early returns from the usable? function and does not require rb-fsevent.

      def self.usable?
        version = RbConfig::CONFIG['target_os'][OS_REGEXP, :major_version]
        return false unless version
        return true if version.to_i >= 13
        # ^--- HERE

        require 'rb-fsevent'
        ...

[coveralls]

Coverage increased (+0.5%) to 98.154% when pulling 37e9536 on jirutka:fix-rb-fsevent-2 into b653060 on guard:master.

[rymai]

Thanks @jirutka!