Explicitly use python2 and pip2

Author: azuresol

.travis.yml

@@ -31,8 +31,8 @@ before_install:
   - sudo apt-get -q update
 
 install:
-  - sudo apt-get install -y python python3 python3-pip
-  - pip install grpcio scapy codecov
+  - sudo apt-get install -y python2.7 python3 python3-pip
+  - pip2 install grpcio scapy codecov
   - pip3 install grpcio coverage
   - "[[ ${DEBUG:-0} == 0 ]] || sudo apt-get install -y g++-5"  # install gcov-5
   - "[[ ${SANITIZE:-0} == 0 ]] || sudo apt-get install -y llvm-3.8"