Use googletest master branch.

We recommend something newer than googletest 1.7.0 because
we want to pick up a fix for an RTTI issue related to mocking
google/googletest#610
That's fixed on the googletest master branch after 2015-11-10.

README.md

@@ -54,9 +54,7 @@ Shaderc into.
 ```sh
 git clone https://github.com/google/shaderc $SOURCE_DIR
 cd $SOURCE_DIR/third_party
-git clone -b release-1.7.0 https://github.com/google/googlemock.git gmock-1.7.0
-git clone -b release-1.7.0 https://github.com/google/googletest.git \
-    gmock-1.7.0/gtest
+git clone https://github.com/google/googletest.git
 git clone https://github.com/google/glslang glslang
 cd $SOURCE_DIR/
 ```

third_party/CMakeLists.txt

@@ -1,8 +1,8 @@
 # Suppress all warnings from third-party projects.
 set_property(DIRECTORY APPEND PROPERTY COMPILE_OPTIONS -w)
 
-if (IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/gmock-1.7.0)
-  add_subdirectory(gmock-1.7.0)
+if (IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/googletest)
+  add_subdirectory(googletest)
 endif()
 if (NOT TARGET gmock)
   message(FATAL_ERROR "gmock was not found - required for tests")