[Dubbo-2129]Enable checkstyle and rat plugin in travis

[chickenlj]

What is the purpose of the change

Enable checkstyle and rat plugin in travis, fix #2129

Brief changelog

./mvnw clean package -Dcheckstyle.skip=false -Drat.skip=false

Verifying this change

Travis check license header and code style automatically.

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[codecov-io]

Codecov Report

Merging #2130 into master will increase coverage by 0.09%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #2130      +/-   ##
============================================
+ Coverage     54.03%   54.12%   +0.09%     
- Complexity     5072     5079       +7     
============================================
  Files           559      559              
  Lines         24922    24922              
  Branches       4442     4442              
============================================
+ Hits          13466    13489      +23     
+ Misses         9411     9394      -17     
+ Partials       2045     2039       -6

Impacted Files	Coverage Δ	Complexity Δ
...ubbo/rpc/protocol/dubbo/ChannelWrappedInvoker.java	37.5% <0%> (-12.5%)	3% <0%> (ø)
...he/dubbo/remoting/transport/netty/NettyServer.java	67.85% <0%> (-3.58%)	8% <0%> (-1%)
...ache/dubbo/remoting/transport/mina/MinaClient.java	56.92% <0%> (-1.54%)	8% <0%> (-1%)
.../src/main/java/org/apache/dubbo/rpc/RpcStatus.java	57.89% <0%> (-1.06%)	18% <0%> (ø)
...he/dubbo/registry/multicast/MulticastRegistry.java	57.74% <0%> (+1.67%)	41% <0%> (+3%)	⬆️
...e/dubbo/remoting/transport/netty4/NettyServer.java	75.4% <0%> (+3.27%)	10% <0%> (+1%)	⬆️
.../dubbo/remoting/transport/netty4/NettyChannel.java	66.25% <0%> (+6.25%)	22% <0%> (+1%)	⬆️
...onfig/spring/extension/SpringExtensionFactory.java	85.71% <0%> (+7.14%)	10% <0%> (ø)	⬇️
...e/dubbo/remoting/transport/netty/NettyChannel.java	66.25% <0%> (+8.75%)	21% <0%> (+2%)	⬆️
...che/dubbo/remoting/transport/mina/MinaChannel.java	53.52% <0%> (+11.26%)	16% <0%> (+1%)	⬆️
... and 1 more

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