Remove support for VS 2013.

Docs/GettingStarted.dox

@@ -703,7 +703,7 @@ To build:
 rake make [<platform>] [<option>=<value> [<option>=<value>]] [[<platform>_]build_tree=/path/to/build-tree] [numjobs=n] [clean_first] [unfilter]
 \endverbatim
 
-The default <generator> when not specified is 'generic', which let CMake to detect and choose what generator is available in the host to use. The possible values are: 'codeblocks', 'eclipse', 'ninja', 'vs2013', 'vs2015', 'vs2017', 'xcode'.
+The default <generator> when not specified is 'generic', which let CMake to detect and choose what generator is available in the host to use. The possible values are: 'codeblocks', 'eclipse', 'ninja', 'vs2015', 'vs2017', 'xcode'.
 
 The default <platform> when not specified is 'native'. The possible values are: 'android', 'web', 'ios', 'tvos', 'mingw', 'rpi'. Naturally this influences the compiler toolchain being used in the generated build tree.
 

Rakefile

@@ -62,7 +62,7 @@ task :cmake do
     case option
     when 'cmake', 'generic'
       # do nothing
-    when 'clean', 'codeblocks', 'codelite', 'eclipse', 'ninja', 'vs2013', 'vs2015', 'vs2017', 'xcode'
+    when 'clean', 'codeblocks', 'codelite', 'eclipse', 'ninja', 'vs2015', 'vs2017', 'xcode'
       script = "cmake_#{option}" unless script == 'cmake_clean'
     when 'android', 'arm', 'ios', 'tvos', 'mingw', 'rpi', 'web'
       platform = option
@@ -103,7 +103,7 @@ task :make do
   ARGV.each { |option|
     task option.to_sym do ; end; Rake::Task[option].clear   # No-op hack
     case option
-    when 'codeblocks', 'codelite', 'eclipse', 'generic', 'make', 'ninja', 'vs2013', 'vs2015', 'vs2017', 'xcode'
+    when 'codeblocks', 'codelite', 'eclipse', 'generic', 'make', 'ninja', 'vs2015', 'vs2017', 'xcode'
       # do nothing
     when 'android', 'arm', 'ios', 'tvos', 'mingw', 'rpi', 'web'
       platform = option