Fix migration script generation

In the 0.18.3 release the script to migrate from 0.18.2 was missing.
This will generate a new version 0.18.4 that when install will generate
scripts to migrate from all old versions to it, so it will be possible
to migrate existing users to 0.18.4 (but not to 0.18.3)

Makefile

@@ -1,7 +1,7 @@
 # cartodb/Makefile
 
 EXTENSION = cartodb
-EXTVERSION = 0.18.3
+EXTVERSION = 0.18.4
 
 SED = sed
 
@@ -75,7 +75,9 @@ UPGRADABLE = \
   0.17.1 \
   0.18.0 \
   0.18.1 \
+  0.18.2 \
   0.18.3 \
+  0.18.4 \
   $(EXTVERSION)dev \
   $(EXTVERSION)next \
   $(END)