Fix libcoreir-python build on travis

[leonardt]

See branch libcoreir-python-travis for WIP. Currently blocked by linking -lpython3.7m inside the manylinux environment.

[leonardt]

Latest build demonstrating the issue: https://travis-ci.org/leonardt/pycoreir/builds/517553347

Specific error:

$ docker exec manylinux bash -c "cd pycoreir/libcoreir-python && ./run_travis_test.sh"
/usr/local/gcc-4.9.0/bin/g++-4.9.0 -I/pycoreir/coreir-cpp/include -std=c++11 -Wall -fPIC -Werror -I./include/ -I/opt/_internal/cpython-3.7.3/include/python3.7m -I/opt/_internal/cpython-3.7.3/include/python3.7m -DPYTHON_HOME="L\"/opt/_internal/cpython-3.7.3\"" -DPYTHON_EXECUTABLE="L\"/opt/python/cp37-cp37m/bin/python\"" -c -o build/libcoreir-python.o coreir-python.cpp
/usr/local/gcc-4.9.0/bin/g++-4.9.0 -L/pycoreir/coreir-cpp/lib -lcoreir -L /opt/_internal/cpython-3.7.3/lib/python3.7/config-3.7m-x86_64-linux-gnu -lpython3.7m -lcrypt -lpthread -ldl  -lutil -lrt -lm  -shared -o build/libcoreir-python.so build/libcoreir-python.o 
/opt/rh/devtoolset-2/root/usr/bin/ld: cannot find -lpython3.7m
collect2: error: ld returned 1 exit status
make: *** [build/libcoreir-python.so] Error 1
/usr/local/gcc-4.9.0/bin/g++-4.9.0 -L/pycoreir/coreir-cpp/lib -lcoreir -L /opt/_internal/cpython-3.7.3/lib/python3.7/config-3.7m-x86_64-linux-gnu -lpython3.7m -lcrypt -lpthread -ldl  -lutil -lrt -lm  -shared -o build/libcoreir-python.so build/libcoreir-python.o 
/opt/rh/devtoolset-2/root/usr/bin/ld: cannot find -lpython3.7m
collect2: error: ld returned 1 exit status
make: *** [build/libcoreir-python.so] Error 1
The command "docker exec manylinux bash -c "cd pycoreir/libcoreir-python && ./run_travis_test.sh"" exited with 2.