Fix a compilation error in version 4.0.0beta
https://github.com/gmarcais/mummer/issues/7

diff -ru mummer-4.0.0beta.orig/src/essaMEM/mummer.cpp mummer-4.0.0beta/src/essaMEM/mummer.cpp
--- mummer-4.0.0beta.orig/src/essaMEM/mummer.cpp	2017-04-06 19:49:04.000000000 +0200
+++ mummer-4.0.0beta/src/essaMEM/mummer.cpp	2017-05-17 15:36:51.998784593 +0200
@@ -218,7 +218,7 @@
       case 22:
 #ifdef VERSION
         std::cout << VERSION << '\n';
-#elif
+#else
         std::cout << "<unknown version>\n";
 #endif
         exit(0);
