Fix `script/test` in a non-Travis environment

`script/test` will now only attempt to call `script/mirror` if either
TRAVIS_PULL_REQUEST or TRAVIS_COMMIT_RANGE is set.

script/test

@@ -6,14 +6,14 @@ set -e
 STATUS=0
 bats -t test || STATUS="$?"
 
-if [ "$TRAVIS_PULL_REQUEST" != "false" ] && [ -z "$TRAVIS_COMMIT_RANGE" ]; then
+if [ "${TRAVIS_PULL_REQUEST:-false}" != "false" ] &&  [ -z "$TRAVIS_COMMIT_RANGE" ]; then
   first_sha="$(curl -fsSL https://github.com/${TRAVIS_REPO_SLUG}/pull/${TRAVIS_PULL_REQUEST}.patch | head -1 | awk '{print $2}')"
   export TRAVIS_COMMIT_RANGE="${first_sha}^..${TRAVIS_COMMIT}"
 fi
 
 if [ "$TRAVIS_SECURE_ENV_VARS" = "true" ]; then
   ./script/mirror update "$TRAVIS_COMMIT_RANGE"
-elif [ "$TRAVIS_PULL_REQUEST" != "false" ]; then
+elif [ "${TRAVIS_PULL_REQUEST:-false}" != "false" ]; then
   ./script/mirror verify "$TRAVIS_COMMIT_RANGE"
 fi