Use int instead of string for bitbucket response.

[lkysow]

Based on https://docs.atlassian.com/bitbucket-server/rest/5.15.0/bitbucket-rest.html the nextPageStart field should be an int.

[codecov]

Codecov Report

Merging #354 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #354   +/-   ##
=======================================
  Coverage   70.67%   70.67%           
=======================================
  Files          61       61           
  Lines        3655     3655           
=======================================
  Hits         2583     2583           
  Misses        893      893           
  Partials      179      179

Impacted Files	Coverage Δ
server/events/vcs/bitbucketserver/client.go	38.09% <100%> (ø)	⬆️

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[kipkoan]

I confirmed that Bitbucket is returning an int for the nextPageStart field:

curl .../rest/api/1.0/projects/anatf/repos/zoml-platform/pull-requests/1/changes?start=0

...
,"size":25,"isLastPage":false,"start":0,"limit":25,"nextPageStart":25}