building: util: Read data for cache digest in chunks.

This fixes MemoryError when hashing big files:
  File "c:\python27\lib\site-packages\PyInstaller\building\utils.py", line 333, in cacheDigest
    data = open(fnm, "rb").read()
MemoryError

PyInstaller/building/utils.py

@@ -338,8 +338,10 @@ def checkCache(fnm, strip=False, upx=False, dist_nm=None):
 
 
 def cacheDigest(fnm, redirects):
-    data = open(fnm, "rb").read()
-    hasher = hashlib.md5(data)
+    hasher = hashlib.md5()
+    with open(fnm, "rb") as f:
+        for chunk in iter(lambda: f.read(16 * 1024), b""):
+            hasher.update(chunk)
     if redirects:
         redirects = str(redirects)
         if is_py3: