travis_ci: run tests for 4.9 kernel

Make sure that libbpf sanitizes BTF properly for older kernels.
Add a stage for 4.9.0 kernel in TravisCI.
For now make test failures non-blocking by adding 4.9.0 to `allow_failures`
section.
Blacklist is copy-pasted 5.5.0 kernel blacklist.

.travis.yml

@@ -91,6 +91,11 @@ jobs:
           language: bash
           script: sudo $CI_ROOT/managers/ubuntu.sh || travis_terminate 1
 
+        - name: Kernel 4.9.0 + selftests
+          language: bash
+          env: KERNEL=4.9.0
+          script:  $CI_ROOT/vmtest/run_vmtest.sh || travis_terminate 1
+
         - name: Kernel 5.5.0 + selftests
           language: bash
           env: KERNEL=5.5.0
@@ -121,3 +126,5 @@ jobs:
               - sudo apt-get install -y libelf-dev pkg-config
           script:
               - scripts/coverity.sh || travis_terminate 1
+    allow_failures:
+        - env: KERNEL=4.9.0