upgrade gtest to 1.7 and show stderr for failed tests (fix #726)

Patch proposed to upstream in https://github.com/tanakh/waf-unittest/pull/9
Merge pull request #748 from jubatus/update-unittest

Update unittest_gtest.py
Add empty-mix test for clustering
Fix SEGV on mixing gmm compressive storage
Implement mixable_holder::clear_each_mixables
Add clear() to server_base
Modify result type of server_base::clear
Add clear to clustering
Regenerate
Increment revision when clear
Add test for clear
Add clear_mine for clustering::storage
Clear gmm state when empty data is passed in gmm::batch
Clear k-means when empty data is passed
Add test for clearing clustering
Output log when clear