install libsodium for travis

.travis.yml

@@ -1 +1,6 @@
+sudo: true
 language: crystal
+install:
+  - ./travis-install-lib-sodium.sh
+before_script:
+  - export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/lib

travis-install-lib-sodium.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+# The purpose of this file is to install libsodium in
+# the Travis CI environment. Outside this environment,
+# you would probably not want to install it like this.
+#
+# from: google/hat-backup
+
+set -e
+
+wget https://download.libsodium.org/libsodium/releases/libsodium-stable-2018-02-14.tar.gz
+tar xvfz libsodium-stable-2018-02-14.tar.gz
+cd libsodium-stable
+sudo ./configure
+sudo make
+sudo make install