fix new shellcheck errors

.travis.yml

@@ -40,7 +40,7 @@ script:
   - if [ "$TRAVIS_OS_NAME" = "linux" ]; then ~/shfmt -l -w -i 2 . ; fi
   - if [ "$TRAVIS_OS_NAME" = "linux" ]; then git diff --exit-code && echo "shfmt OK" ; fi
   - if [ "$TRAVIS_OS_NAME" = "linux" ]; then shellcheck -V ; fi
-  - if [ "$TRAVIS_OS_NAME" = "linux" ]; then shellcheck **/*.sh && echo "shellcheck OK" ; fi
+  - if [ "$TRAVIS_OS_NAME" = "linux" ]; then shellcheck -e SC2181 **/*.sh && echo "shellcheck OK" ; fi
   - cd ..
   - git clone https://github.com/Neilpang/acmetest.git && cp -r acme.sh acmetest/ && cd acmetest
   - if [ "$TRAVIS_OS_NAME" = "linux" -a "$NGROK_TOKEN" ]; then sudo TEST_LOCAL="$TEST_LOCAL" NGROK_TOKEN="$NGROK_TOKEN" ./letest.sh ; fi

dnsapi/dns_aws.sh

@@ -208,7 +208,7 @@ aws_rest() {
   kServiceH="$(printf "$Service%s" | _hmac "$Hash" "$kRegionH" hex)"
   _debug2 kServiceH "$kServiceH"
 
-  kSigningH="$(printf "aws4_request%s" | _hmac "$Hash" "$kServiceH" hex)"
+  kSigningH="$(printf "%s" "aws4_request" | _hmac "$Hash" "$kServiceH" hex)"
   _debug2 kSigningH "$kSigningH"
 
   signature="$(printf "$StringToSign%s" | _hmac "$Hash" "$kSigningH" hex)"