Fix build failure

[biyuhao]

Fix build failure, error message:

[FATAL] Non-resolvable parent POM for org.apache.dubbo.samples:dubbo-spring-boot-registry-nacos-consumer-sample:[unknown-version]: Failure to find org.apache.dubbo.samples:dubbo-spring-boot-registry-nacos-samples:pom:2.7.1 in http://repo.maven.apache.org/maven2 was cached in the local repository, resolution will not be reattempted until the update interval of central has elapsed or updates are forced and 'parent.relativePath' points at wrong local POM @ line 21, column 13
 @ 
[ERROR] The build could not read 1 project -> [Help 1]
[ERROR]   
[ERROR]   The project org.apache.dubbo.samples:dubbo-spring-boot-registry-nacos-consumer-sample:[unknown-version] (/home/travis/build/biyuhao/incubator-dubbo-spring-boot-project/dubbo-spring-boot-samples/dubbo-registry-nacos-samples/consumer-sample/pom.xml) has 1 error
[ERROR]     Non-resolvable parent POM for org.apache.dubbo.samples:dubbo-spring-boot-registry-nacos-consumer-sample:[unknown-version]: Failure to find org.apache.dubbo.samples:dubbo-spring-boot-registry-nacos-samples:pom:2.7.1 in http://repo.maven.apache.org/maven2 was cached in the local repository, resolution will not be reattempted until the update interval of central has elapsed or updates are forced and 'parent.relativePath' points at wrong local POM @ line 21, column 13 -> [Help 2]

replace const version with ${revision}

[codecov-io]

Codecov Report

Merging #504 into master will decrease coverage by 0.32%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #504      +/-   ##
============================================
- Coverage     80.06%   79.74%   -0.33%     
  Complexity      201      201              
============================================
  Files            39       39              
  Lines           617      617              
  Branches         58       58              
============================================
- Hits            494      492       -2     
- Misses           99      101       +2     
  Partials         24       24

Impacted Files	Coverage Δ	Complexity Δ
...ntext/event/AwaitingNonWebApplicationListener.java	77.41% <0%> (-3.23%)	24% <0%> (ø)

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