recommend 1.46 since that dodges the ubuntu issue with interprocess -…

… refs mapnik#950 and mapnik#1001

INSTALL.md

@@ -37,9 +37,10 @@ Build dependencies are:
 Mapnik Core depends on:
 
  * Boost
-    - >= 1.47 is recommended
-    - >= 1.45.x is required if compiling with clang++
-    - >= 1.42.x works on most systems and most compilers
+    - >= 1.46 is recommended
+    - >= 1.45 is required if compiling with clang++
+    - >= 1.42 works on most systems and most compilers
+    - >= 1.47 is required to support wkt/wkb geometry output (optional)
     - These libraries are required:
       - filesystem
       - system