Use MemoizedZipManifests for all operations. Fixes #240.

CHANGES.txt

@@ -2,6 +2,17 @@
 CHANGES
 =======
 
+---
+5.7
+---
+
+* Issue #240: Based on real-world performance measures against 5.4, zip
+  manifests are now cached in all circumstances. The
+  ``PKG_RESOURCES_CACHE_ZIP_MANIFESTS`` environment variable is no longer
+  relevant. The observed "memory increase" referenced in the 5.4 release
+  notes and detailed in Issue #154 was likely not an increase over the status
+  quo, but rather only an increase over not storing the zip info at all.
+
 ---
 5.6
 ---

pkg_resources.py

@@ -1605,11 +1605,7 @@ class ZipProvider(EggProvider):
     """Resource support for zips and eggs"""
 
     eagers = None
-    _zip_manifests = (
-        MemoizedZipManifests()
-        if os.environ.get('PKG_RESOURCES_CACHE_ZIP_MANIFESTS') else
-        ZipManifests()
-    )
+    _zip_manifests = MemoizedZipManifests()
 
     def __init__(self, module):
         EggProvider.__init__(self, module)