fix ruby 2.3, 2.4 syntax errors.
```text
/home/travis/build/mokhan/net-hippie/lib/net/hippie.rb:11:in `require': /home/travis/build/mokhan/net-hippie/lib/net/hippie/client.rb:77: syntax error, unexpected keyword_rescue, expecting keyword_end (SyntaxError)
rescue *::Net::Hippie::CONNECTION_ERRORS => error
^
/home/travis/build/mokhan/net-hippie/lib/net/hippie/client.rb:77: dynamic constant assignment
rescue *::Net::Hippie::CONNECTION_ERRORS => error
^
/home/travis/build/mokhan/net-hippie/lib/net/hippie/client.rb:77: syntax error, unexpected =>, expecting '='
rescue *::Net::Hippie::CONNECTION_ERRORS => error
^
from /home/travis/build/mokhan/net-hippie/lib/net/hippie.rb:11:in `<top (required)>'
from /home/travis/build/mokhan/net-hippie/test/test_helper.rb:5:in `require'
from /home/travis/build/mokhan/net-hippie/test/test_helper.rb:5:in `<top (required)>'
from /home/travis/build/mokhan/net-hippie/test/net/api_test.rb:1:in `require'
from /home/travis/build/mokhan/net-hippie/test/net/api_test.rb:1:in `<top (required)>'
from /home/travis/build/mokhan/net-hippie/vendor/bundle/ruby/2.3.0/gems/rake-10.5.0/lib/rake/rake_test_loader.rb:15:in `require'
from /home/travis/build/mokhan/net-hippie/vendor/bundle/ruby/2.3.0/gems/rake-10.5.0/lib/rake/rake_test_loader.rb:15:in `block in <main>'
from /home/travis/build/mokhan/net-hippie/vendor/bundle/ruby/2.3.0/gems/rake-10.5.0/lib/rake/rake_test_loader.rb:4:in `select'
from /home/travis/build/mokhan/net-hippie/vendor/bundle/ruby/2.3.0/gems/rake-10.5.0/lib/rake/rake_test_loader.rb:4:in `<main>'
rake aborted!
Command failed with status (1): [ruby -I"lib:test:lib" -I"/home/travis/build/mokhan/net-hippie/vendor/bundle/ruby/2.3.0/gems/rake-10.5.0/lib" "/home/travis/build/mokhan/net-hippie/vendor/bundle/ruby/2.3.0/gems/rake-10.5.0/lib/rake/rake_test_loader.rb" "test/net/api_test.rb" "test/net/client_test.rb" "test/net/content_type_mapper_test.rb" "test/net/hippie_test.rb" ]
/home/travis/.rvm/rubies/ruby-2.3.8/bin/bundle:30:in `block in <main>'
/home/travis/.rvm/rubies/ruby-2.3.8/bin/bundle:22:in `<main>'
/home/travis/.rvm/gems/ruby-2.3.8@global/bin/ruby_executable_hooks:24:in `eval'
/home/travis/.rvm/gems/ruby-2.3.8@global/bin/ruby_executable_hooks:24:in `<main>'
Tasks: TOP => test
(See full trace by running task with --trace)
```