Use URI.parse.open/File.open instead of open

This commit updates the dashboard to use `URI.parse` instead of `Kernel#open`
because open can potentially open files on the server and has other
possible security issues.

Also updated the logger to use `File.open` as it is more explicit and
doesn't have the extra potential for abuse like `Kernel#open`

https://rubocop.readthedocs.io/en/latest/cops_security/#securityopen

lib/vmpooler/api/dashboard.rb

@@ -83,7 +83,7 @@ module Vmpooler
             history ||= {}
 
             begin
-              buffer = open(graph_link('.ready.*&from=-1hour&format=json')).read
+              buffer = URI.parse(graph_link('.ready.*&from=-1hour&format=json')).read
               history = JSON.parse(buffer)
 
               history.each do |pool|
@@ -136,7 +136,7 @@ module Vmpooler
         if params[:history]
           if graph_url
             begin
-              buffer = open(graph_link('.running.*&from=-1hour&format=json')).read
+              buffer = URI.parse(graph_link('.running.*&from=-1hour&format=json')).read
               JSON.parse(buffer).each do |pool|
                 if pool['target'] =~ /.*\.(.*)$/
                   pool['name'] = Regexp.last_match[1]

lib/vmpooler/logger.rb

@@ -16,7 +16,7 @@ module Vmpooler
 
       puts "[#{stamp}] #{string}" if ENV['VMPOOLER_DEBUG']
 
-      open(@file, 'a') do |f|
+      File.open(@file, 'a') do |f|
         f.puts "[#{stamp}] #{string}"
       end
     end