Fixed year pattern.

Year was matching any digit, one or more times. This could lead to way
too eager matching.

Match years as either a group of 2, or a group of 4 digits.

Was already committed to https://github.com/logstash/grok-patterns/commit/6c9bd3b85ea03acb83d945e4b48e9f66956b553a, but not merged here.