cache: Bump minor version to 6

Needed for the previous change

apt-pkg/pkgcache.cc

@@ -59,7 +59,7 @@ pkgCache::Header::Header()
    /* Whenever the structures change the major version should be bumped,
       whenever the generator changes the minor version should be bumped. */
    APT_HEADER_SET(MajorVersion, 10);
-   APT_HEADER_SET(MinorVersion, 5);
+   APT_HEADER_SET(MinorVersion, 6);
    APT_HEADER_SET(Dirty, false);
 
    APT_HEADER_SET(HeaderSz, sizeof(pkgCache::Header));