Github user astroshim commented on the issue:
https://github.com/apache/zeppelin/pull/1193
The reason of CI fails is..
```
Resolving deltas: 100% (13946/13946), completed with 385 local objects.
From https://github.com/apache/zeppelin
* branch refs/pull/1193/merge -> FETCH_HEAD
travis_time:end:16c30e18:start=1471009118100481571,finish=1471009120921906857,duration=2821425286
[0K$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout
[0Ktravis_fold:start:apt
[0K[33;1mAdding APT Sources (BETA)[0m
$ export DEBIAN_FRONTEND=noninteractive
travis_time:start:2224b420
[0K$ curl -sSL
"http://keyserver.ubuntu.com/pks/lookup?op=get&search=0x51716619E084DAB9"; |
sudo -E apt-key add -
gpg: no valid OpenPGP data found.
travis_time:end:2224b420:start=1471009120981996375,finish=1471009141163566540,duration=20181570165
[0K
[31;1mThe command "curl -sSL
"http://keyserver.ubuntu.com/pks/lookup?op=get&search=0x51716619E084DAB9"; |
sudo -E apt-key add -" failed and exited with 2 during .[0m
Your build has been stopped.
```
so I re-trigger CI.
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