Fix marking branch for removal after merge in Gitlab

[WouldYouKindly]

Hi.

I renamed should_remove_source_branch to remove_source_branch, and flag is now correctly set on merge request.

Tested on GitLab Community Edition 11.9.4

According to https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/api/merge_requests.md#L745

[codecov[bot]]

Codecov Report

Merging #348 into master will increase coverage by 0.28%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #348      +/-   ##
==========================================
+ Coverage   93.92%   94.20%   +0.28%     
==========================================
  Files          11       11              
  Lines        1103     1157      +54     
==========================================
+ Hits         1036     1090      +54     
  Misses         67       67              

Impacted Files	Coverage Δ
pyup/providers/gitlab.py	85.24% <100.00%> (+0.37%)	:arrow_up:
pyup/providers/github.py	99.37% <0.00%> (-0.01%)	:arrow_down:
pyup/updates.py	100.00% <0.00%> (ø)
pyup/config.py	98.88% <0.00%> (+0.01%)	:arrow_up:
pyup/bot.py	97.88% <0.00%> (+0.04%)	:arrow_up:
pyup/pullrequest.py	97.22% <0.00%> (+0.44%)	:arrow_up:
pyup/requirements.py	88.46% <0.00%> (+0.96%)	:arrow_up:

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[rafaelpivato]

Thanks for showing where the problem was, @WouldYouKindly :+1:

[WouldYouKindly]

@rafaelpivato thanks for taking the time to fix it!