Fix format CI. (#453)

Author: g-easy

tools/travis/check_format.sh

@@ -19,6 +19,7 @@ set -x
 wget -O $HOME/bin/buildifier https://github.com/bazelbuild/buildtools/releases/download/2.2.1/buildifier
 chmod +x $HOME/bin/buildifier
 # Install cmake-format.
-pip install --user 'cmake_format>=0.5.2'
+pyenv global 3.7.1
+pip3 install --user 'cmake_format>=0.5.2'
 # Check format.
 tools/format.sh