Enhancement: Do not trigger Travis CI/AppVeyor tests for document only changes in PR

[tshradheya]

Considering that a lot of students use the AB4 for the CS2103 project and the Travis tests take up to 5-6 minutes, it is not a good idea to make the tests run when the student has made document changes.

This will also prevent high traffic for Travis builds which was experienced previous semester.

Credit for idea: TEAMMATES/teammates#6432

[yamgent]

Nice suggestion, thanks for raising it up.

I checked the build script and I think we can exclude these gradle tasks for documentation-only changes:

• checkstyleMain
• checkstyleTest
• headless allTests
• coverage
• coveralls

I chose to exclude only these few specific tasks, because the rest serves non-tests purposes (for example, ./gradlew asciidoctor actually generates the documentation files for web deployment, run-checks.sh runs checks on files that are not codes as well, and etc...)