Bump to Rails 5.1.1

[ferblape]

Connects to #680

What does this PR do?

This PR updates Gobierto to Rails 5.1.1

[codecov-io]

Codecov Report

Merging #726 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #726      +/-   ##
==========================================
+ Coverage   76.16%   76.16%   +<.01%     
==========================================
  Files         306      306              
  Lines        7443     7444       +1     
==========================================
+ Hits         5669     5670       +1     
  Misses       1774     1774

Impacted Files	Coverage Δ
app/forms/user/notification_form.rb	100% <100%> (ø)	⬆️
app/models/site.rb	96.51% <100%> (+0.04%)	⬆️

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