Update ChageLog for new 4.2 version numbering.

Author: Joe Hamman

src/ChangeLog

@@ -26,7 +26,7 @@ Usage:
 
 
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-***** Description of changes between VIC 4.2.1 and VIC 4.2.2 *****
+***** Description of changes between VIC 4.2.a and VIC 4.2.b *****
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 Bug Fixes:
@@ -49,7 +49,7 @@ Fixed memory error in initialize atmos when OUTPUT_FORCE = TRUE.
 
 
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-***** Description of changes between VIC 4.2.0 and VIC 4.2.1 *****
+***** Description of changes between VIC 4.2 and VIC 4.2.a *****
 -------------------------------------------------------------------------------
 
 Bug Fixes: