Fix compat with RubyInstaller-2.4 on Windows (brianmario#875)

Since RubyInstaller-2.4+ is bundled with MSYS2 and the libmariadbclient
can be installed per gemspec library dependency, it is easy to build
the mysql2 gem in Windows.

The MSYS2/MINGW dependency feature is documented here:
https://github.com/oneclick/rubyinstaller2/wiki/For-gem-developers#msys2-library-dependency

This also adds ruby-2.4 binaries, so that the mysql2 is still usabel as
a binary gem.

Fixes brianmario#861

The change in the spec is required for mariadbclient. It throws an error
if no query was executed.

Due to the stdcall convention on i686, the mysql_query() function check
fails, so that it is omitted, now.

Signed-off-by: Jun Aruga <[email protected]>

spec/mysql2/statement_spec.rb

@@ -731,7 +731,6 @@ def stmt_count
 
     it 'should return number of rows affected by an insert' do
       stmt = @client.prepare 'INSERT INTO lastIdTest (blah) VALUES (?)'
-      expect(stmt.affected_rows).to eq 0
       stmt.execute 1
       expect(stmt.affected_rows).to eq 1
     end