fix travis build (web-test-suite)

Travis failed continuously because the project `grails-test-suite-web` was configured to run with 4 executors using the JVM memory setting `-Xmx1024M`. IMHO this easily exceeds the Travis worker memory configuration which is set to 3 GB. Reducing the number of parallel test runs to *2* and lowering the memory footprint by changing the JVM memory setting to *`-Xmx768M`* will fix this problem.

Another issues was that the `grails-tests-suite-web` project took way to long to complete. Running all tests  takes more than 10mins which unfortunately is the configured timeout for console inactivity. Until someone finds a better solution after every test suite run a `.` is logged. This happens only if the environment variable `TRAVIS` is set.

In addition there is the new task `createCombinedReport ` which creates a combined test report for both test tasks (`test`, `execIsolatedTests`).