update PINCache (#17)

* use newer PINCache to avoid runtime error

Older PINCache causes runtime erorr sometimes.
To avoid this problem, set version of PINCache.

refs: TextureGroup/Texture#769

* use PINCache 3.0.1-beta.6 or later

with submodule reference

Author: fumito-ito

Cartfile

@@ -1,3 +1,3 @@
-
 github "RxSwiftCommunity/RxDataSources"
 github "texturegroup/texture"
+github "pinterest/PINCache" ~> "3.0.1-beta.6"

Cartfile.resolved

@@ -5,3 +5,4 @@ github "pinterest/PINCache" "3.0.1-beta.5"
 github "pinterest/PINOperation" "1.0.3"
 github "pinterest/PINRemoteImage" "3.0.0-beta.12"
 github "texturegroup/texture" "2.5.1"
+github "pinterest/PINCache" "3.0.1-beta.6"