Bump version to 1.6.19

Merge pull request HangfireIO#1156 from HangfireIO/release-1.6.18

Release 1.6.18

Add a note regarding recurring jobs and DST handling

Bump version to 1.6.17

Add release notes for 1.6.16

Bump version to 1.6.15, add release notes

Bump version to 1.6.14

Update release notes for 1.6.13

Add release notes for 1.6.12

Fix NullReferenceException when User is null for ASP.NET Core also