Fix zone line parsing with range values

This is a fix for a critical issue with time zones that have end dates with ranges like the `Apr Sun>=1 3:00` below in the America\Chihuahua zone.

```
# Chihuahua (near US border)
Zone America/Ojinaga	-6:57:40 -	LMT	1922 Jan 1 0:02:20
			-7:00	-	MST	1927 Jun 10 23:00
			-6:00	-	CST	1930 Nov 15
			-7:00	-	MST	1931 May  1 23:00
			-6:00	-	CST	1931 Oct
			-7:00	-	MST	1932 Apr  1
			-6:00	-	CST	1996
			-6:00	Mexico	C%sT	1998
			-6:00	-	CST	1998 Apr Sun>=1 3:00
			-7:00	Mexico	M%sT	2010
			-7:00	US	M%sT
```