Add net-http gem

[ldodds]

Since adding premailer-rails I've been getting these warnings from rails, rails console, rspec, etc:

/usr/share/rvm/rubies/ruby-2.7.6/lib/ruby/2.7.0/net/protocol.rb:66: warning: already initialized constant Net::ProtocRetryError
/home/ldodds/.rvm/gems/ruby-2.7.6/gems/net-protocol-0.2.1/lib/net/protocol.rb:68: warning: previous definition of ProtocRetryError was here
/usr/share/rvm/rubies/ruby-2.7.6/lib/ruby/2.7.0/net/protocol.rb:206: warning: already initialized constant Net::BufferedIO::BUFSIZE
/home/ldodds/.rvm/gems/ruby-2.7.6/gems/net-protocol-0.2.1/lib/net/protocol.rb:214: warning: previous definition of BUFSIZE was here
/usr/share/rvm/rubies/ruby-2.7.6/lib/ruby/2.7.0/net/protocol.rb:503: warning: already initialized constant Net::NetPrivate::Socket
/home/ldodds/.rvm/gems/ruby-2.7.6/gems/net-protocol-0.2.1/lib/net/protocol.rb:541: warning: previous definition of Socket was here

There are various suggestions that adding net-http gem will fix this. Problem appears due to some dependency issues.

PR explicitly adds net-http gem to our Gemfile.