travis: execute build actions on the "script" step

Otherwise a default actions for this step will be executed:

$ ./configure && make && make test
/home/travis/build.sh: line 45: ./configure: No such file or directory
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

.travis.yml

@@ -7,5 +7,5 @@ env:
   - TR_ARCH=x86_64
   - TR_ARCH=armv7hf
   - TR_ARCH=aarch64
-before_install:
+script:
   - sudo make -C scripts/build $TR_ARCH