Remove support for PHP5

PHP5 is no longer actively supported by the PHP developers and security
support for it ends completely at the end of 2018, so it doesn't make
sense to include support for it in the upcoming SWIG 4.0.0 release.

See #701.

Source/Modules/main.cxx

@@ -65,7 +65,7 @@ static const char *usage1 = (const char *) "\
      -copyctor       - Automatically generate copy constructors wherever possible\n\
      -cpperraswarn   - Treat the preprocessor #error statement as #warning (default)\n\
      -cppext <ext>   - Change file extension of generated C++ files to <ext>\n\
-                       (default is cxx, except for PHP5 which uses cpp)\n\
+                       (default is cxx)\n\
      -copyright      - Display copyright notices\n\
      -debug-classes  - Display information about the classes found in the interface\n\
      -debug-module <n>- Display module parse tree at stages 1-4, <n> is a csv list of stages\n\