Reduce travis load by replacing an exclusive grep with an inclusive grep (#10964)

* add -n to avoid compiling

* switch to an include rather than exclude strategy

util/travis_test.sh

@@ -2,8 +2,6 @@
 
 source util/travis_utils.sh
 
-NUM_CORE_CHANGES=$(echo "$QMK_CHANGES" | grep -Ecv -e '^(docs/)' -e '^(keyboards/)' -e '^(layouts/)' -e '^(util/)' -e '^(lib/python/)' -e '^(bin/qmk)' -e '^(requirements.txt)' -e '(.travis.yml)')
-
 if [[ "$TRAVIS_COMMIT_MESSAGE" == *"[skip test]"* ]]; then
     echo "Skipping due to commit message"
     exit 0