bump test tensorflow version to 2.1

Author: njzjz

.travis.yml

@@ -65,12 +65,12 @@ matrix:
       env:
       - CC=gcc-5
       - CXX=g++-5
-      - TENSORFLOW_VERSION=2.0
+      - TENSORFLOW_VERSION=2.1
     - python: 3.7
       env:
       - CC=gcc-8
       - CXX=g++-8
-      - TENSORFLOW_VERSION=2.0
+      - TENSORFLOW_VERSION=2.1
 before_install:
   - pip install --upgrade pip
   - pip install --upgrade setuptools