Updates dependencies to use github.com/mattermost/mattermost-server/v5

[hanzei]

Summary

A community member did run into an issue using github.com/mattermost/mattermost-server instead of github.com/mattermost/mattermost-server/v5. This PR updated the dependencies to use github.com/mattermost/mattermost-server/v5.

Ticket Link

None

[codecov-commenter]

Codecov Report

Merging #61 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #61   +/-   ##
=======================================
  Coverage   29.48%   29.48%           
=======================================
  Files           8        8           
  Lines         234      234           
=======================================
  Hits           69       69           
  Misses        153      153           
  Partials       12       12           

Impacted Files	Coverage Δ
server/plugin.go	4.83% <ø> (ø)
server/util/util.go	40.67% <ø> (ø)

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[jfrerich]

LGTM! Thanks, @hanzei

[hanzei]

Skipping QA review as it's only a dependency update