fix travis build (web-test-suite)

- adjust number of parallel executors
- decreased memory jvm settings
- restructure/cleanup test execution
Merge pull request #527 from JoergFiedler/2.4-fix-travis-ci-build

fix travis build (web-test-suite)
Merge branch '2.4.x'

Conflicts:
	grails-test-suite-web/build.gradle