try to update ubuntu version for newer gcc

[MOON-CLJ]

No description provided.

[tclh123]

Maybe install g++-7 and don't modify other things such as CMakeLists.txt.

[MOON-CLJ]

@tclh123 also add CMAKE_C_COMPILER CMAKE_CXX_COMPILER in CMakeLists.txt is because cmake build gtest seems like not use CC and CXX，see https://travis-ci.org/douban/libmc/jobs/483223951。

-- The C compiler identification is Clang 5.0.0
-- The CXX compiler identification is Clang 5.0.0

In file included from /home/travis/build/douban/libmc/build/googletest-src/googletest/include/gtest/internal/gtest-port.h:253:
In file included from /usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/type_traits:35:
/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/bits/c++0x_warning.h:32:2: error: 

[MOON-CLJ]

@tclh123 take a look https://docs.travis-ci.com/user/build-matrix/ ， i dont use "matrix:\n include" way to specific gcc version as https://docs.travis-ci.com/user/languages/cpp/#c11c11-and-beyond-and-toolchain-versioning 's suggestion，because i dont distinguish how to combine "matrix: \n include" (include only) and normal matrix。

[MOON-CLJ]

i dont distinguish how to combine "matrix: \n include" (include only) and normal matrix。

i dont distinguish how to build matrix to contain following dimensions instead of use "matrix: \n include" explicitly list every combination one by one 。

1, use "matrix: \n include" to specific clang & gcc version
2, two python version
3，env release and debug

[tclh123]

@MOON-CLJ #91 lgty?