Unable to mark an MkRelease with prerelease=true

See #1363, release log:

• c6b49d9 by @llorllale: Merge branch 'master' of https...
• 5383b03 by @llorllale: (#1400) User.notifications() n...
• d0240c1 by @llorllale: (#1399) Randomized port used f...
• 5ded5a2 by @amihaiemil: Merge pull request #1391 from ...
• 485406e by @amihaiemil: Merge pull request #1398 from ...
• b850683 by @amihaiemil: Merge pull request #1396 from ...
• 54ac983 by @amihaiemil: Merge pull request #1397 from ...
• d87c3c4 by @llorllale: (#1304) Fixing appveyor build ...
• f5c9591 by @llorllale: (#1394) Randomized port used f...
• eb76cf3 by @llorllale: (#1390) Fixed Release.Smart.pr...
• babeb3c by @llorllale: (#1395) Fixed more checkstyle ...
• e594734 by @llorllale: (#1395) Fixed checkstyle issue...
• 40e09a2 by @llorllale: (#1304) Implemented RtUser.mar...
• 8ee8cb9 by @llorllale: Merge branch 'master' of https...
• 5d8c7e6 by @amihaiemil: Merge pull request #1393 from ...
• 9247e33 by @amihaiemil: Merge pull request #1392 from ...
• d110c7d by @llorllale: (#1388) Refactored more tests ...
• 5bccfc0 by @llorllale: (#1384) Now using 'Existence' ...
• 710a461 by @llorllale: (#1304) Implemented RtUser.mar...
• 939d24d by @amihaiemil: Merge pull request #1386 from ...
• and 6 more...

Released by Rultor 1.67.3, see build log