RFR: Add support for Jira 8

[Robin Westberg]

Use non deprecated method of getting available fields which is available in Jira 8.

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Commit messages:
 - Use non deprecated method of getting available fields for Jira 8

Changes: https://git.openjdk.java.net/skara/pull/994/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=994&range=00
  Stats: 60 lines in 1 file changed: 33 ins; 21 del; 6 mod
  Patch: https://git.openjdk.java.net/skara/pull/994.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/994/head:pull/994

PR: https://git.openjdk.java.net/skara/pull/994