.travis.yml: Build and test strict stackless build.

Previously, testing of stackless build happened (manually) in
travis-stackless branch. However, stackless offers important
featureset, so it's worth to test it as a part of the main
CI. Strict stackless is used because it's the "real" stackless
build, which avoids using C stack as much as possible (non-strict
just prefers heap over C stack, but may end up using the latter).

.travis.yml

@@ -61,6 +61,11 @@ script:
   # run coveralls coverage analysis (try to, even if some builds/tests failed)
   - (cd ports/unix && coveralls --root ../.. --build-root . --gcov $(which gcov) --gcov-options '\-o build-coverage/' --include py --include extmod)
 
+  # run tests on stackless build
+  - rm -rf ports/unix/build-coverage
+  - make -C ports/unix coverage CFLAGS_EXTRA="-DMICROPY_STACKLESS=1 -DMICROPY_STACKLESS_STRICT=1"
+  - (cd tests && MICROPY_CPYTHON3=python3.4 MICROPY_MICROPYTHON=../ports/unix/micropython_coverage ./run-tests)
+
 after_failure:
   - (cd tests && for exp in *.exp; do testbase=$(basename $exp .exp); echo -e "\nFAILURE $testbase"; diff -u $testbase.exp $testbase.out; done)
   - (grep "FAIL" ports/qemu-arm/build/console.out)