Use puma as development web server

It seems to be the default in Rails 5 (we even have a configuration file
for it under `config/puma.rb`) and Capybara seems to need it to run
system specs in Rails 5.1:

https://github.com/teamcapybara/capybara/#setup

Gemfile

@@ -72,6 +72,7 @@ group :development, :test do
   gem "knapsack_pro", "~> 1.1.0"
   gem "launchy", "~> 2.4.3"
   gem "letter_opener_web", "~> 1.3.4"
+  gem "puma", "~> 3.12.1"
   gem "spring", "~> 2.0.1"
   gem "spring-commands-rspec", "~> 1.0.4"
 end

Gemfile.lock

@@ -345,6 +345,7 @@ GEM
       arel (>= 6)
     powerpack (0.1.2)
     public_suffix (3.0.3)
+    puma (3.12.1)
     rack (2.0.6)
     rack-accept (0.4.5)
       rack (>= 0.4)
@@ -581,6 +582,7 @@ DEPENDENCIES
   paranoia (~> 2.4.1)
   pg (~> 0.21.0)
   pg_search (~> 2.0.1)
+  puma (~> 3.12.1)
   rails (= 5.0.7.2)
   rails-assets-leaflet!
   rails-assets-markdown-it (~> 8.2.1)!