Remove Net::SSH dependency for Ruby 1.9.3

Given Ruby 1.9.3 unsupported status, we do not want to limit fog
applications by limiting to older versions of `net-ssh`

If running on Ruby 1.9.3, please ensure your application is declaring
it's own dependency in it's `Gemfile`:

    gem "net-ssh", "< 3.0"

fog-core.gemspec

@@ -23,7 +23,7 @@ Gem::Specification.new do |spec|
   spec.add_dependency("formatador", "~> 0.2")
   spec.add_dependency("mime-types")
   spec.add_dependency("net-scp", "~> 1.1")
-  spec.add_dependency("net-ssh", ">= 2.1.3", "< 3.0")
+  spec.add_dependency("net-ssh", ">= 2.1.3")
 
   spec.add_development_dependency("coveralls")
   spec.add_development_dependency("minitest")