Changes:
1. Since travis-ci.org is terminated, we are moving CI of this repo to PingCAP internal jenkins CI. Removing all travis related scripts.
2. remove `./rust-toolchain` file, and `Make build` will download the file from latest TiKV master, so we can keep in sync with TiKV.
3. fixing flaky test_auto_roll_max_size_info_log. In the background rocksdb can trigger log rotation. If log rotates exactly when the test check for LOG file existence, the check can fail. Removing the check.
Signed-off-by: Yi Wu <yiwu@pingcap.com>