Try to build libsodium on docker

.travis.yml

+sudo: false
 language: python
 python:
   - "3.4"
-# command to install dependencies
+before_install:
+ - bash ci/libsodium.sh
 install:
   - pip install -r requirements.txt
 # command to run tests

ci/libsodium.sh

+#!/bin/bash
+set -ev
+
+git clone git://github.com/jedisct1/libsodium.git
+cd libsodium
+git checkout tags/1.0.3
+./autogen.sh
+if [[ $TRAVIS_OS_UNAME = 'Darwin' ]]; then
+  export CFLAGS="-Os -m32 -arch i386";
+  export LDFLAGS="-m32 -arch i386";
+fi
+./configure
+make && make install
+if [[ $TRAVIS_OS_UNAME = 'Linux' ]]; then ldconfig; fi
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