Correctly updated version to 2.4.0 in git_version.cpp

My previous attempt to do this contained a mistake, and I already pushed
it, so the 2.4.0 tag will have to point to this commit instead.

src/core/git_version.cpp

@@ -15,6 +15,6 @@ const int MINOR_VERSION = 4;
 const int PATCH_VERSION = 0;
 const int COMMITS_AHEAD_OF_VERSION = 0;
 const char* VERSION_STRING = "2.4.0";
-const char* VERSION_STRING_PLUS_COMMITS = "2.3.0+0";
+const char* VERSION_STRING_PLUS_COMMITS = "2.4.0+0";
 
 }