Update .travis.yml

* Use GoogleTest for unittest
* Run tests for duck/memory.c
Add CMakeLists.txt

* Use GoogleTest for unit tests
* Compile test program
  * run_memory_tests
Add memory_test.cpp
Rename "namespace" to "ref_namespace"

* Avoid to conflict with the C++ keyword
Merge pull request #12 from gregtour/unittest

Use GoogleTest for testing