Merge pull request #2709 from ethereum/updateChangelog

Update changelog.

Changelog.md

@@ -1,4 +1,4 @@
-### 0.4.15 (unreleased)
+### 0.4.15 (2017-08-08)
 
 Features:
  * Type Checker: Show unimplemented function if trying to instantiate an abstract class.

docs/bugs_by_version.json

@@ -350,6 +350,10 @@
         ], 
         "released": "2017-07-31"
     }, 
+    "0.4.15": {
+        "bugs": [], 
+        "released": "2017-08-08"
+    }, 
     "0.4.2": {
         "bugs": [
             "DelegateCallReturnValue",