Fixing code.google.com links to point to github instead (#2235)

cpp/README

@@ -52,7 +52,7 @@ Requirements:
     $ sudo apt-get install cmake-curses-gui
 
   - Protocol Buffers
-    http://code.google.com/p/protobuf/
+    http://github.com/google/protobuf/
     Version 2.4 or more recent is required (this is available by default for
     recent Debian-based GNU/Linux distributions).
 
@@ -73,13 +73,13 @@ Requirements:
     $ ./configure && make && sudo make install
 
   - Google Test
-    http://code.google.com/p/googletest/
+    http://github.com/google/googletest
 
     Installation:
     $ sudo apt-get install libgtest-dev
 
   - RE2
-    http://code.google.com/p/re2/
+    http://github.com/google/re2
 
     Installation:
     $ sudo apt-get install libre2-dev