repair broken download links ( zookeeper for Mac)

Modify url of boost and zookeeper, this commit may repair issue jegonzal#176 :  broken download links (Yosemite Mac OS X): zookeeper?

CMakeLists.txt

@@ -183,7 +183,7 @@ ExternalProject_Add(libbz2
 # unordered containers.
 ExternalProject_Add(boost
   PREFIX ${GraphLab_SOURCE_DIR}/deps/boost
-  URL "http://sourceforge.net/projects/boost/files/boost/1.53.0/boost_1_53_0.tar.gz"
+  URL "http://tcpdiag.dl.sourceforge.net/project/boost/boost/1.53.0/boost_1_53_0.tar.gz" 
   URL_MD5 57a9e2047c0f511c4dfcf00eb5eb2fbb
   BUILD_IN_SOURCE 1
   CONFIGURE_COMMAND
@@ -386,8 +386,8 @@ endif()
 
 ExternalProject_Add(zookeeper
   PREFIX ${GraphLab_SOURCE_DIR}/deps/zookeeper
-  URL http://apache.cs.utah.edu/zookeeper/zookeeper-3.4.6/zookeeper-3.4.6.tar.gz
-  URL_MD5 971c379ba65714fd25dc5fe8f14e9ad1
+  URL http://mirror.metrocast.net/apache/zookeeper/zookeeper-3.5.1-alpha/zookeeper-3.5.1-alpha.tar.gz
+  URL_MD5 d85f9751724d3f20f792803b61c4db24
   PATCH_COMMAND ${CMAKE_COMMAND} -E copy_directory ${GraphLab_SOURCE_DIR}/patches/zookeeper/ <SOURCE_DIR>
   BUILD_IN_SOURCE 1
   CONFIGURE_COMMAND ./configure --prefix=<INSTALL_DIR> --disable-shared