add removeInvitation and listInvitation methods to repositoryClient

[ainhoaL]

Add functionality to delete an invitation to join a repo and to list all invitations in a repo.

[codecov]

Codecov Report

Merging #113 (eed348f) into master (d0c05a4) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #113      +/-   ##
============================================
+ Coverage     73.06%   73.22%   +0.15%     
- Complexity      245      247       +2     
============================================
  Files            39       39              
  Lines           880      885       +5     
  Branches         39       39              
============================================
+ Hits            643      648       +5     
  Misses          212      212              
  Partials         25       25              

Impacted Files	Coverage Δ
...va/com/spotify/github/v3/clients/GitHubClient.java	76.15% <100.00%> (+0.10%)	⬆️
...om/spotify/github/v3/clients/RepositoryClient.java	70.86% <100.00%> (+0.94%)	⬆️

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