Add -std=c++11 for python setup on kokoro

[BSBandme]

Kokoro is using c++ 4.8, which needs explicitly declare "-std=c++11" arg

[xfxyjwf]

Jie, can you review this change?

[anandolee]

Do you know why our tests on Kokoro can pass without "-std=c++11" arg

[BSBandme]

some part of the log:

x86_64-linux-gnu-gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fPIC -I. -I../src -I/usr/include/python2.7 -c google/protobuf/pyext/message_factory.cc -o build/temp.linux-x86_64-2.7/google/protobuf/pyext/message_factory.o -Wno-write-strings -Wno-invalid-offsetof -Wno-sign-compare
cc1plus: warning: command line option ‘-Wstrict-prototypes’ is valid for C/ObjC but not for C++ [enabled by default]
In file included from /usr/include/c++/4.8/type_traits:35:0,
                 from ../src/google/protobuf/message.h:115,
                 from ../src/google/protobuf/dynamic_message.h:45,
                 from google/protobuf/pyext/message_factory.cc:33:
/usr/include/c++/4.8/bits/c++0x_warning.h:32:2: error: #error This file requires compiler and library support for the ISO C++ 2011 standard. This support is currently experimental, and must be enabled with the -std=c++11 or -std=gnu++11 compiler options.
 #error This file requires compiler and library support for the \
  ^
In file included from ../src/google/protobuf/arena.h:51:0,
                 from ../src/google/protobuf/message.h:118,
                 from ../src/google/protobuf/dynamic_message.h:45,
                 from google/protobuf/pyext/message_factory.cc:33:
../src/google/protobuf/arena_impl.h:311:3: warning: identifier ‘static_assert’ is a keyword in C++11 [-Wc++0x-compat]
   static_assert(kBlockHeaderSize % 8 == 0,
   ^

I think it's because of on kokoro c++ is 4.8, which g++ doesn't support C++11 features by default.