drop GTK_CHECK_VERSION(2,6,0) macro in gtk2/rbgtk*.c

As discussed in issue #182, I dropped GTK_CHECK_VERSION(2,6,0) macro in gtk2.

* This pull request contains file changes related to GTK_CHECK_VERSION(2,6,0) only in gtk2/rbgtk*.c.

* This is another part of drop GTK_CHECK_VERSION(2,6,0) macro in gtk2.

* Because ruby-gtk2 requires GTK+ 2.6.x to install, I also updated related READMEs.

* Sorry, I forgot to commit gtk2/rbgdkdisplay.c. I added it in this pull request.