vim.rb: enhance the logic of vim.rev. (#139)

* vim.rb: enhance the logic of vim.rev.

1. No matter if rev is specified when calls RbVmomi.connect, vim.rev
   should be real version that effects current VIM connection.

   Without this fix, say, vSphere is 6.5, and we give 7.0 to
   RbVmomi.connect, then returned vim.rev will be 7.0. Even if we
   give 100.0, obviously 100.0 is not a reasonable vSphere version,
   returned vim.rev will be 100.0.

   With this fix, returned vim.rev will be min value of vSphere
   version and specified rev.

2. Enhanced version comparison.

   Without this fix, version comparison is done by string comparison,
   as a result "100.0" is smaller than "6.0". vSphere version has
   reached to 8.x, once it reaches to "10.0", this bug will be
   triggered.

   With this fix, a version number is split into parts by ".", then
   compare each part from left to right, so that "10.0" is bigger
   than "6.0".

* Revert "vim.rb: enhance the logic of vim.rev."

This reverts commit 4efb0be.

* vim.rb: enhance the logic of vim.rev.

1. No matter if rev is specified when calls RbVmomi.connect, vim.rev
   should be real version that effects current VIM connection.

   Without this fix, say, vSphere is 6.5, and we give 7.0 to
   RbVmomi.connect, then returned vim.rev will be 7.0. Even if we
   give 100.0, obviously 100.0 is not a reasonable vSphere version,
   returned vim.rev will be 100.0.

   With this fix, returned vim.rev will be min value of vSphere
   version and specified rev.

2. Enhanced version comparison.

   Without this fix, version comparison is done by string comparison,
   as a result "100.0" is smaller than "6.0". vSphere version has
   reached to 8.x, once it reaches to "10.0", this bug will be
   triggered.

   With this fix, a version number is split into parts by ".", then
   compare each part from left to right, so that "10.0" is bigger
   than "6.0".

lib/rbvmomi/vim.rb

@@ -39,8 +39,7 @@ def self.connect opts
     opts[:port] ||= (opts[:ssl] ? 443 : 80)
     opts[:path] ||= '/sdk'
     opts[:ns] ||= 'urn:vim25'
-    rev_given = opts[:rev] != nil
-    opts[:rev] = '6.5' unless rev_given
+    opts[:rev] = '6.5' if opts[:rev].nil?
     opts[:debug] = (!ENV['RBVMOMI_DEBUG'].empty? rescue false) unless opts.member? :debug
 
     conn = new(opts).tap do |vim|
@@ -61,10 +60,8 @@ def self.connect opts
             vim.serviceContent.sessionManager.Login :userName => opts[:user], :password => opts[:password]
         end
       end
-      unless rev_given
-        rev = vim.serviceContent.about.apiVersion
-        vim.rev = [rev, '6.5'].min
-      end
+      rev = vim.serviceContent.about.apiVersion
+      vim.rev = [rev, opts[:rev]].min { |a, b| Gem::Version.new(a) <=> Gem::Version.new(b) }
     end
 
     at_exit { conn.close }