Use libsodium 1.0.7

Because *something* is now demanding it.

https://travis-ci.org/travis-ci/php-src-builder/jobs/491742270#L2791

Author: BanzaiMan

bin/install-libsodium

@@ -11,10 +11,10 @@ fi
 
 LIBSODIUM_INSTALL_DIR=$HOME/.phpenv/versions/$VERSION
 
-git clone -b stable https://github.com/jedisct1/libsodium.git
-
+curl -sfSLO --retry 20 https://github.com/jedisct1/libsodium/releases/download/1.0.7/libsodium-1.0.7.tar.gz
+tar xzf libsodium-1.0.7.tar.gz
 # compile
-pushd libsodium
+pushd libsodium-1.0.7
 ./configure --prefix=$LIBSODIUM_INSTALL_DIR
 make check
 make install