Use shallow clone to speed up git clone on Travis CI

Shallow clone is faster than a normal one, there is no need to clone the
whole history of a repository when we only needs its latest or certain
state of commit.

.travis.yml

@@ -28,7 +28,7 @@ script:
   - if [ "$TRAVIS_OS_NAME" = "linux" ]; then shellcheck -V ; fi
   - if [ "$TRAVIS_OS_NAME" = "linux" ]; then shellcheck -e SC2181 **/*.sh && echo "shellcheck OK" ; fi
   - cd ..
-  - git clone https://github.com/Neilpang/acmetest.git && cp -r acme.sh acmetest/ && cd acmetest
+  - git clone --depth 1 https://github.com/Neilpang/acmetest.git && cp -r acme.sh acmetest/ && cd acmetest
   - if [ "$TRAVIS_OS_NAME" = "linux" -a "$NGROK_TOKEN" ]; then sudo TEST_LOCAL="$TEST_LOCAL" NGROK_TOKEN="$NGROK_TOKEN" ./rundocker.sh testplat ubuntu:latest ; fi
   - if [ "$TRAVIS_OS_NAME" = "osx" -a "$NGROK_TOKEN" ]; then sudo TEST_LOCAL="$TEST_LOCAL" NGROK_TOKEN="$NGROK_TOKEN" ACME_OPENSSL_BIN="$ACME_OPENSSL_BIN" ./letest.sh ; fi