Suppress warning

[swamp09]

This PR suppresses the following warning.

ruby -v
ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-darwin16]

RUBYOPT=-w bundle exec rspec
/Users/numata/ruby/validates_email_format_of/lib/validates_email_format_of.rb:12: warning: character class has duplicated range: /[\!\#\$\%\&\'\*\-\/\=\?\+\-\^\_\`\{\|\}\~]/
/Users/numata/ruby/validates_email_format_of/lib/validates_email_format_of.rb:110: warning: character class has duplicated range: /[\!\#\$\%\&\'\*\-\/\=\?\+\-\^\_\`\{\|\}\~]/

[alexdunae]

Thanks!