Atlantis version in footer

[lkysow]

Followup for #57. Fixes the formatting and tests and adds version to the footer.

Thanks @psalaberria002! Feel free to open up a PR to add your name to the credits! https://github.com/runatlantis/atlantis/blob/master/README.md#credits

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@d8569d3). Click here to learn what that means.
The diff coverage is 80%.

@@           Coverage Diff            @@
##             master     #64   +/-   ##
========================================
  Coverage          ?   63.9%           
========================================
  Files             ?      40           
  Lines             ?    1892           
  Branches          ?       0           
========================================
  Hits              ?    1209           
  Misses            ?     621           
  Partials          ?      62

Impacted Files	Coverage Δ
cmd/version.go	0% <0%> (ø)
server/server.go	70.08% <100%> (ø)
cmd/server.go	75.92% <100%> (ø)

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