getdeps: fix boost download URL

Summary:
Not sure if this is a transient issue, but the URL we've been using to obtain boost has started to return 403 errors.
Switch to a sourceforge download link instead.

Refs: https://github.com/facebook/watchman/pull/797

Reviewed By: chadaustin

Differential Revision: D20739351

fbshipit-source-id: 47483c675d59201a410c9d2a8f6db0f63ea5da69

build/fbcode_builder/manifests/boost

@@ -2,11 +2,11 @@
 name = boost
 
 [download.not(os=windows)]
-url = https://dl.bintray.com/boostorg/release/1.69.0/source/boost_1_69_0.tar.bz2
+url = https://versaweb.dl.sourceforge.net/project/boost/boost/1.69.0/boost_1_69_0.tar.bz2
 sha256 = 8f32d4617390d1c2d16f26a27ab60d97807b35440d45891fa340fc2648b04406
 
 [download.os=windows]
-url = https://dl.bintray.com/boostorg/release/1.69.0/source/boost_1_69_0.zip
+url = https://versaweb.dl.sourceforge.net/project/boost/boost/1.69.0/boost_1_69_0.zip
 sha256 = d074bcbcc0501c4917b965fc890e303ee70d8b01ff5712bae4a6c54f2b6b4e52
 
 [build]