gtest.rb: use gtests internal tuple library

Force gtest to use its internal tuple library.

Fixes build error: 
"./include/gtest/internal/gtest-port.h:449:10: fatal error: 'tr1/tuple' file not found
#include <tr1/tuple>  // NOLINT"
Merge pull request #5 from GeeF/patch-1

gtest.rb: use gtests internal tuple library