travis-tests: set CRIU_PMC_OFF conditionally

We only needed it for kernel 3.19. Apparently, Ubuntu 14.04.5 comes
with a kernel from 16.04 (i.e. 4.4), so we can disable this workaround!

Anyway, just in case, let's do it conditionally.

While at it, slightly improve the comment.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

scripts/travis/travis-tests

@@ -53,10 +53,11 @@ time make CC="$CC" -j4 -C test/zdtm
 
 umask 0000
 export SKIP_PREP=1
-# The 3.19 Ubuntu kernel has a bug. When pagemap are read for a few vma-s
-# for one read call, it returns incorrect data.
-# https://github.com/xemul/criu/issues/207
-export CRIU_PMC_OFF=1
+# The 3.19 kernel (from Ubuntu 14.04) has a bug. When /proc/PID/pagemap
+# is read for a few VMAs in one read call, incorrect data is returned.
+# See https://github.com/xemul/criu/issues/207
+# Kernel 4.4 (from Ubuntu 14.04.5 update) fixes this.
+uname -r | grep -q ^3\.19 && export CRIU_PMC_OFF=1
 
 chmod 0777 test/
 chmod 0777 test/zdtm/static