Fix a few files that claimed to be distributed under GPLv3 by mistake.

* src/eltlparse/Makefile.am, src/ltlvisit/randomltl.cc,
src/ltlvisit/simplify.cc, src/ltlvisit/snf.cc, src/ltlvisit/snf.hh:
Fix GPL to version 2 or later.

src/eltlparse/Makefile.am

@@ -5,7 +5,7 @@
 ##
 ## Spot is free software; you can redistribute it and/or modify it
 ## under the terms of the GNU General Public License as published by
-## the Free Software Foundation; either version 3 of the License, or
+## the Free Software Foundation; either version 2 of the License, or
 ## (at your option) any later version.
 ##
 ## Spot is distributed in the hope that it will be useful, but WITHOUT

src/ltlvisit/randomltl.cc

@@ -9,7 +9,7 @@
 //
 // Spot is free software; you can redistribute it and/or modify it
 // under the terms of the GNU General Public License as published by
-// the Free Software Foundation; either version 3 of the License, or
+// the Free Software Foundation; either version 2 of the License, or
 // (at your option) any later version.
 //
 // Spot is distributed in the hope that it will be useful, but WITHOUT

src/ltlvisit/simplify.cc

@@ -5,7 +5,7 @@
 //
 // Spot is free software; you can redistribute it and/or modify it
 // under the terms of the GNU General Public License as published by
-// the Free Software Foundation; either version 3 of the License, or
+// the Free Software Foundation; either version 2 of the License, or
 // (at your option) any later version.
 //
 // Spot is distributed in the hope that it will be useful, but WITHOUT

src/ltlvisit/snf.cc

@@ -6,7 +6,7 @@
 //
 // Spot is free software; you can redistribute it and/or modify it
 // under the terms of the GNU General Public License as published by
-// the Free Software Foundation; either version 3 of the License, or
+// the Free Software Foundation; either version 2 of the License, or
 // (at your option) any later version.
 //
 // Spot is distributed in the hope that it will be useful, but WITHOUT

src/ltlvisit/snf.hh

@@ -6,7 +6,7 @@
 //
 // Spot is free software; you can redistribute it and/or modify it
 // under the terms of the GNU General Public License as published by
-// the Free Software Foundation; either version 3 of the License, or
+// the Free Software Foundation; either version 2 of the License, or
 // (at your option) any later version.
 //
 // Spot is distributed in the hope that it will be useful, but WITHOUT