Add travis ci config

.travis.yml

+language: python
+
+python:
+  - '2.7'
+  - '3.5'
+  - '3.6'
+
+script:
+  - if [[ $TRAVIS_PYTHON_VERSION != 2.7 ]]; then /bin/bash ./test/ci/check_code_style.sh; fi
+  - /bin/bash ./test/ci/test_download.sh
+
+notifications:
+  email:
+    on_success: change
+    on_failure: always

test/ci/check_code_style.sh

+#!/bin/bash
+function abort(){
+    echo "Your change doesn't follow PaddleHub's code style." 1>&2
+    echo "Please use pre-commit to check what is wrong." 1>&2
+    exit 1
+}
+
+trap 'abort' 0
+set -e
+
+cd $TRAVIS_BUILD_DIR
+export PATH=/usr/bin:$PATH
+pre-commit install
+
+if ! pre-commit run -a ; then
+    git diff
+    exit 1
+fi
+
+trap : 0

test/ci/test_download_dataset.sh

+#!/bin/bash
+set -o errexit
+
+base_path=$(cd `dirname $0`/../..; pwd)
+cd $base_path
+
+python dataset/download_pet.py