Use quiver directly, and update README.google. This is so that

we don't have to fork quiver_hashcode as well.

BUG=
R=whesse@google.com

Review-Url: https://codereview.chromium.org/2944463002 .

README.google

@@ -9,4 +9,5 @@ Description:
 This directory contains the tuple package as checked
 out from git@github.com:kseo/tuple.git at tag v1.0.1
 on June 13, 2017.
-
+Modified to use quiver.core directly instead of
+quiver_hashcode.

lib/tuple.dart

@@ -22,6 +22,6 @@
 /// ```
 library tuple;
 
-import 'package:quiver_hashcode/hashcode.dart';
+import 'package:quiver/core.dart';
 
 part 'package:tuple/src/tuple.dart';

pubspec.yaml

@@ -8,6 +8,6 @@ homepage: https://github.com/kseo/tuple
 environment:
   sdk: '>=1.6.0'
 dependencies:
-  quiver_hashcode: '>=1.0.0-dev.1 <1.1.0'
+  quiver: '>=0.22.0 <0.23.0'
 dev_dependencies:
   test: any