travis: turn on alpine build

Cc: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

.travis.yml

@@ -9,6 +9,7 @@ env:
   - TR_ARCH=armv7hf
   - TR_ARCH=aarch64
   - TR_ARCH=ppc64le
+  - TR_ARCH=alpine
 script:
   - sudo make -C scripts/travis $TR_ARCH
 after_success: