ci: honor RUN_TESTS flag on Travis

[yejingyan]

Fixes #95.

The Travis matrix defines RUN_TESTS=true globally, but the Khronos ICD jobs used RUN_TEST=false while also having no explicit script: section. Because Travis defaults to running mvn test, those jobs could still execute the test suite and skip OpenCL-dependent tests even though the job was intended to build without running tests.

This patch:

• renames the matrix overrides to RUN_TESTS=false, matching the global variable name;
• adds an explicit Travis script: section that runs mvn test only when RUN_TESTS=true;
• runs mvn -DskipTests package for the non-test Khronos ICD jobs.

Validation:

• mvn -q -DskipTests package
• grep -n "RUN_TEST" .travis.yml confirms only RUN_TESTS is used.