Enable checkstyle and rat plugin for travis CI

[diecui1202]

What is the purpose of the change

Enable checkstyle and rat plugin for travis CI

Brief changelog

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Verifying this change

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[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@b4db875). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #2393   +/-   ##
=========================================
  Coverage          ?   54.75%           
  Complexity        ?     5235           
=========================================
  Files             ?      573           
  Lines             ?    25456           
  Branches          ?     4537           
=========================================
  Hits              ?    13939           
  Misses            ?     9422           
  Partials          ?     2095

Impacted Files	Coverage Δ	Complexity Δ
.../protocol/hessian/HttpClientConnectionFactory.java	76.92% <ø> (ø)	3 <0> (?)

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