Fix broken builds by specifying Ubuntu trusty dist.

[lightningrob]

Reference: https://travis-ci.community/t/error-installing-oraclejdk8-expected-feature-release-number-in-range-of-9-to-14-but-got-8/3766

[jsjtzyy]

Thanks for this fix. Will rebase existing PRs.

[cgtz]

I can merge once travis passes

[zzmao]

Thanks for making the change!

[codecov-io]

Codecov Report

Merging #1231 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1231      +/-   ##
============================================
- Coverage     69.58%   69.54%   -0.05%     
+ Complexity     5553     5552       -1     
============================================
  Files           432      432              
  Lines         33943    33943              
  Branches       4342     4342              
============================================
- Hits          23619    23604      -15     
- Misses         9131     9140       +9     
- Partials       1193     1199       +6

Impacted Files	Coverage Δ	Complexity Δ
...b.ambry.network/BlockingChannelConnectionPool.java	69.48% <0%> (-2.82%)	7% <0%> (-1%)
...java/com.github.ambry.store/CompactionManager.java	87.33% <0%> (-2.67%)	19% <0%> (ø)
...in/java/com.github.ambry.store/BlobStoreStats.java	71.13% <0%> (-0.62%)	103% <0%> (ø)
...ain/java/com.github.ambry.router/PutOperation.java	90.61% <0%> (-0.54%)	111% <0%> (-1%)
...in/java/com.github.ambry.network/SocketServer.java	83.6% <0%> (-0.41%)	15% <0%> (ø)
.../java/com.github.ambry.router/DeleteOperation.java	94.77% <0%> (+1.49%)	45% <0%> (+1%)	⬆️

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