Bump middleman-core version

middleman-blog.gemspec

@@ -18,7 +18,7 @@ Gem::Specification.new do | s |
   s.require_paths         = [ "lib" ]
   s.required_ruby_version = '>= 2.3.0'
 
-  s.add_dependency( "middleman-core", [ ">= 4.0.0" ] )
+  s.add_dependency( "middleman-core", [ "~> 4.3"   ] )
   s.add_dependency( "tzinfo",         [ ">= 0.3.0" ] )
   s.add_dependency( "addressable",    [ "~> 2.3"   ] )