Use ${project.parent.version} instead of ${parent.version}

[jmini]

This PR fixes following Maven warnings:

[WARNING] 
[WARNING] Some problems were encountered while building the effective model for org.openapitools.swagger.parser:swagger-parser-v3:jar:2.0.5-SNAPSHOT
[WARNING] The expression ${parent.version} is deprecated. Please use ${project.parent.version} instead.
[WARNING] 
[WARNING] Some problems were encountered while building the effective model for org.openapitools.swagger.parser:swagger-parser-v2-converter:jar:2.0.5-SNAPSHOT
[WARNING] The expression ${parent.version} is deprecated. Please use ${project.parent.version} instead.
[WARNING] The expression ${parent.version} is deprecated. Please use ${project.parent.version} instead.
[WARNING] 
[WARNING] Some problems were encountered while building the effective model for org.openapitools.swagger.parser:swagger-parser:jar:2.0.5-SNAPSHOT
[WARNING] The expression ${parent.version} is deprecated. Please use ${project.parent.version} instead.
[WARNING] The expression ${parent.version} is deprecated. Please use ${project.parent.version} instead.
[WARNING] 
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING] 
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
[WARNING] 

[gracekarina]

@jmini Thanks!