Add ssl_security_level support

This change adds an :ssl_security_level option to change the security
level of the SSLContext before Excon uses it to create an SSLSocket.

OpenSSL 1.1.0 introduced security levels, where increasingly large
security levels forbid increasingly strong cryptographic algorithms and
parameters.  Security level 0 forbids nothing (the default behavior in
previous versions of OpenSSL), and security level 5 allows only very
strong algorithms and very long key lengths.

Every OpenSSL library now has a default security level compiled into it,
and programs may need to change the security level from its default.

Changing an OpenSSL::SSL::SSLContext's security level requires Ruby
2.4.0+ or version 2.0.0+ of the openssl gem. See the following for more
information:

- https://ruby-doc.org/stdlib-2.4.0/libdoc/openssl/rdoc/OpenSSL/SSL/SSLContext.html#method-i-security_level-3D
- https://www.openssl.org/docs/man1.1.0/man3/SSL_CTX_set_security_level.html

lib/excon/constants.rb

@@ -99,6 +99,7 @@ module Excon
     :ssl_version,
     :ssl_min_version,
     :ssl_max_version,
+    :ssl_security_level,
     :ssl_proxy_headers,
     :ssl_uri_schemes,
     :tcp_nodelay,

lib/excon/ssl_socket.rb

@@ -12,6 +12,11 @@ def initialize(data = {})
       # create ssl context
       ssl_context = OpenSSL::SSL::SSLContext.new
 
+      # set the security level before setting other parameters affected by it
+      if @data[:ssl_security_level]
+        ssl_context.security_level = @data[:ssl_security_level]
+      end
+
       # disable less secure options, when supported
       ssl_context_options = OpenSSL::SSL::SSLContext::DEFAULT_PARAMS[:options]
       if defined?(OpenSSL::SSL::OP_DONT_INSERT_EMPTY_FRAGMENTS)