Disable flaky test/xlog/snap_io_rate.test.lua
Disable flaky test/xlog/snap_io_rate.test.lua

This test fails on Travis CI for some unknown reason.
A ticket has been re-opened, see #1610.

rtsisyk/tarantool,rtsisyk/tarantool,rtsisyk/tarantool,rtsisyk/tarantool