update pvsystem and tracking tutorials for pvlib 0.4 api changes (#281)

* update pvsystem.ipynb

* update tracking tutorial

* update whatsnew

Author: wholmgren

docs/sphinx/source/whatsnew/v0.4.2.txt

@@ -1,6 +1,6 @@
 .. _whatsnew_0420:
 
-v0.4.2 (October 31, 2016)
+v0.4.2 (December 7, 2016)
 -------------------------
 
 This is a minor release from 0.4.1.
@@ -51,8 +51,11 @@ Enhancements
 
 Other
 ~~~~~
+
 * Typical modeling results could change by ~1%, depending on location, if they
   depend on the turbidity table
+* Fixed issues with pvsystem, tracking, and tmy_to_power jupyter notebooks
+  (:issue:`267`, :issue:`273`)
 
 
 Code Contributors
@@ -63,3 +66,4 @@ Code Contributors
 * Volker Beutner
 * Mark Mikofski
 * Marc Anoma
+* Giuseppe Peronato