update release notes for 1.21.4

CHANGELOG.txt

@@ -1,3 +1,28 @@
+2016-07-05 1.21.4:
+------------------
+
+Bug fixes:
+----------
+
+ * Choose newest Pypi skeleton version; fix unicode encoding in pypi metadata #1092
+ * Add Numpy 1.11 to all_versions dict for autocompletion #1078
+ * Fix MSVC 3.3/3.4 builds when Win7SDK not installed #1072
+ * Fix an error with build number, when build number is None or otherwise invalid #1088
+
+Known issues:
+-------------
+
+ * Environment activation requires conda >=4.1.6.  The activate.bat script does not look in the right place for the activate.d folder.
+ * The test suite on Linux and Mac fails the python-build, python-run, and python-build-run tests, because an errant __conda-version__.txt file is somehow present.  It is not clear where it comes from, and each of these tests pass when run individually.  If you have mysterious issues, and you use __conda-version__.txt or files like it, please file an issue.
+
+Contributors:
+-------------
+ * @adament
+ * @aleksey
+ * @ikalev
+ * @msarahan
+
+
 2016-06-27 1.21.3:
 ------------------