Properly test Python wheels in Travis. (#5749)

python/build-wheel-macos.sh

@@ -79,3 +79,5 @@ for ((i=0; i<${#PY_VERSIONS[@]}; ++i)); do
     mv dist/*.whl ../.whl/
   popd
 done
+
+git clean -f -f -x -d -e .whl -e $DOWNLOAD_DIR

python/build-wheel-manylinux1.sh

@@ -1,5 +1,10 @@
 #!/bin/bash
 
+set -x
+
+# Cause the script to exit if a single command fails.
+set -e
+
 cat << EOF > "/usr/bin/nproc"
 #!/bin/bash
 echo 10
@@ -48,6 +53,8 @@ for ((i=0; i<${#PYTHONS[@]}; ++i)); do
   popd
 done
 
+git clean -f -f -x -d -e .whl
+
 # Rename the wheels so that they can be uploaded to PyPI. TODO(rkn): This is a
 # hack, we should use auditwheel instead.
 pushd .whl