* gc.c: fix gloval variable name.
  Now we have following environments (and related variable names).
  * RUBY_GC_HEAP_INIT_SLOTS
  * RUBY_GC_HEAP_FREE_SLOTS
  * RUBY_GC_HEAP_GROWTH_FACTOR (new from 2.1)
  * RUBY_GC_HEAP_GROWTH_MAX_SLOTS (new from 2.1)
  * obsolete
    * RUBY_FREE_MIN       -> RUBY_GC_HEAP_FREE_SLOTS (from 2.1)
    * RUBY_HEAP_MIN_SLOTS -> RUBY_GC_HEAP_INIT_SLOTS (from 2.1)
  * RUBY_GC_MALLOC_LIMIT
  * RUBY_GC_MALLOC_LIMIT_MAX (new from 2.1)
  * RUBY_GC_MALLOC_LIMIT_GROWTH_FACTOR (new from 2.1)
  * RUBY_GC_OLDSPACE_LIMIT (new from 2.1)
  * RUBY_GC_OLDSPACE_LIMIT_MAX (new from 2.1)
  * RUBY_GC_OLDSPACE_LIMIT_GROWTH_FACTOR (new from 2.1)
* test/ruby/test_gc.rb: catch up this change.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43811 b2dd03c8-39d4-4d8f-98ff-823fe69b080e