Fix download path on multiprecision.org

www.multiprecision.org seems to have been re-organized. Update build.conf
to contain a base download URL that works.

Author: nvswarren

build.conf

@@ -3,7 +3,7 @@ BUILD_NUMBER=arduino2
 
 AVR_SOURCES="http://distribute.atmel.no/tools/opensource/Atmel-AVR-GNU-Toolchain/${AVR_VERSION}"
 GNU_SOURCES="http://mirror.switch.ch/ftp/mirror/gnu"
-MPC_SOURCES="http://www.multiprecision.org/mpc/download"
+MPC_SOURCES="http://www.multiprecision.org/downloads"
 
 # The following version numbers are by default parsed out of the SOURCES.README
 # in the Atmel distribution, you can override here if you want (or if it breaks)