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)
```

2 files changed, 2 insertions, 9 deletions

diff --git a/lib/net/hippie.rb b/lib/net/hippie.rb
index 6c39a50..71e786d 100644
--- a/lib/net/hippie.rb
+++ b/lib/net/hippie.rb
@@ -14,14 +14,6 @@ require 'net/hippie/api'
 module Net
   # net/http for hippies.
   module Hippie
-    CONNECTION_ERRORS = [
-      EOFError,
-      Errno::ECONNRESET,
-      Errno::EINVAL,
-      Net::ProtocolError,
-      Timeout::Error
-    ].freeze
-
     def self.logger
       @logger ||= Logger.new(STDOUT)
     end
diff --git a/lib/net/hippie/client.rb b/lib/net/hippie/client.rb
index db31faa..e28633d 100644
--- a/lib/net/hippie/client.rb
+++ b/lib/net/hippie/client.rb
@@ -74,7 +74,8 @@ module Net
         retries = 0 if retries.nil? || retries.negative?
         0.upto(retries) do |n|
           return yield self
-        rescue *::Net::Hippie::CONNECTION_ERRORS => error
+        rescue EOFError, Errno::ECONNRESET, Errno::EINVAL,
+               Net::ProtocolError, Timeout::Error => error
           raise error if n == retries
 
           delay = ((2**n) * 0.1) + Random.rand(0.05) # delay + jitter