Updated Mongolian carrier prefixes (#2118)

* Updated Mongolian carrier prefixes

Updated from Communications Regulatory Commission of Mongolian.
http://www.crc.gov.mn/k/4L/36

Another source: http://www.wtng.info/wtng-976-mn.html

* revert 97683|G-Mobile

resources/carrier/en/976.txt

@@ -18,4 +18,12 @@
 97685|Mobicom
 97686|Unitel
 97688|Unitel
-97689|Unitel
+97689|Unitel
+97690|Skytel
+97691|Skytel
+97693|G-Mobile
+97694|Mobicom
+97695|Mobicom
+97696|Skytel
+97698|G-Mobile
+97699|Mobicom