Updating Boost download URL.

Boost does not use Sourceforge anymore. Instead, their download links
point to dl.bintray.com.

Earlier today Sourceforge was down, and later the Boost downloaded file
checksum did not match, which further reinforces the need to change
the URL.

Dockerfile

@@ -24,7 +24,7 @@ WORKDIR /usr/local
 ARG BOOST_VERSION=1_66_0
 ARG BOOST_VERSION_DOT=1.66.0
 ARG BOOST_HASH=5721818253e6a0989583192f96782c4a98eb6204965316df9f5ad75819225ca9
-RUN curl -s -L -o  boost_${BOOST_VERSION}.tar.bz2 https://sourceforge.net/projects/boost/files/boost/${BOOST_VERSION_DOT}/boost_${BOOST_VERSION}.tar.bz2/download \
+RUN curl -s -L -o  boost_${BOOST_VERSION}.tar.bz2 https://dl.bintray.com/boostorg/release/${BOOST_VERSION_DOT}/source/boost_${BOOST_VERSION}.tar.bz2 \
     && echo "${BOOST_HASH} boost_${BOOST_VERSION}.tar.bz2" | sha256sum -c \
     && tar -xvf boost_${BOOST_VERSION}.tar.bz2 \
     && cd boost_${BOOST_VERSION} \