Fix automerge bug in Bitbucket Server

[lkysow]

BB Server requires we send a 'version' parameter along with our call to
/merge.

Fixes #479

[codecov]

Codecov Report

Merging #484 into master will increase coverage by 0.03%.
The diff coverage is 36.36%.

@@            Coverage Diff             @@
##           master     #484      +/-   ##
==========================================
+ Coverage   70.53%   70.57%   +0.03%     
==========================================
  Files          63       63              
  Lines        4249     4258       +9     
==========================================
+ Hits         2997     3005       +8     
+ Misses       1031     1028       -3     
- Partials      221      225       +4

Impacted Files	Coverage Δ
server/events/command_runner.go	65.46% <0%> (ø)	⬆️
server/events/vcs/bitbucketserver/client.go	33.72% <40%> (+3.1%)	⬆️

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