Upgrade to Version 1.8

[geoffviola]

From https://github.com/google/googletest/releases, it looks like the beta1 gmock and gtest come from version 1.7, but there is a new version 1.8. When compiling gtest.h, GCC emits a warning with the -Wextra flag. The warning was addressed in 1.8: google/googletest#633. I haven't tested that there are no warnings in the 1.8 release.

[dirk-thomas]

Since the latest release 1.8.0 "merges" googletest and googlemock the two separate vendor repos (gtest_vendor and gmock_vendor) should probably be merged in the process of updating to a newer version.