Make bintray test more reliable

[paulmelnikow]

The regex in the bintray service tests is too restrictive:

  2) Bintray
       version
         
	[ GET /asciidoctor/maven/asciidoctorj.json ]:
     ValidationError: child "value" fails because ["value" with value "v1.6.0-RC.1" fails to match the required pattern: /^v\d+(\.\d+)*$/]
      at Object.exports.process (node_modules/joi/lib/errors.js:201:19)
      at internals.Object._validateWithOptions (node_modules/joi/lib/types/any/index.js:751:31)
      at module.exports.internals.Any.root.validate (node_modules/joi/lib/index.js:146:23)
      at Object.pathMatch.matchJSONTypes (node_modules/icedfrisby/lib/pathMatch.js:303:9)
      at _expect (node_modules/icedfrisby/lib/icedfrisby.js:563:10)
      at IcedFrisbyNock._invokeExpects (node_modules/icedfrisby/lib/icedfrisby.js:1261:26)
      at start (node_modules/icedfrisby/lib/icedfrisby.js:1244:12)
      at Request.runCallback [as _callback] (node_modules/icedfrisby/lib/icedfrisby.js:1131:16)
      at Request.self.callback (node_modules/request/request.js:185:22)
      at Request.<anonymous> (node_modules/request/request.js:1161:10)
      at IncomingMessage.<anonymous> (node_modules/request/request.js:1083:12)
      at endReadableNT (_stream_readable.js:1064:12)
      at _combinedTickCallback (internal/process/next_tick.js:139:11)
      at process._tickDomainCallback (internal/process/next_tick.js:219:9)

Ref #1359