Update get_committer_data for latest PyGithub

[DamianZaremba]

v1.55 of PyGithub changed get_emails to return a named tuple (#1890) rather than a dict directly from the decoded API response.

Update the code to access the fields as a property rather than by index & bump the pin so 1.55 is the minimum version.

Fixes #409

[codecov[bot]]

Codecov Report

Merging #410 (3bebe38) into master (17c2384) will increase coverage by 0.08%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #410      +/-   ##
==========================================
+ Coverage   94.24%   94.32%   +0.08%     
==========================================
  Files          11       11              
  Lines        1164     1164              
==========================================
+ Hits         1097     1098       +1     
+ Misses         67       66       -1     

Impacted Files	Coverage Δ
pyup/providers/github.py	99.39% <100.00%> (ø)
pyup/providers/gitlab.py	86.06% <0.00%> (+0.81%)	:arrow_up:

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