Travis: use Ubuntu Bionic so we don't have to compile libsodium

.travis.yml

@@ -1,9 +1,17 @@
+dist: bionic
+
 language: go
 os:
 - linux
 go:
 - 1.x
 
+before_install:
+  - sudo apt-get -y install libsodium
+addons:
+  apt:
+    update: true
+
 script:
 - gimme --list
 - echo $TRAVIS_GO_VERSION
@@ -216,14 +224,6 @@ deploy:
     tags: true
 
 before_deploy:
-- mkdir -p /tmp/bin /tmp/lib /tmp/include
-- export LD_LIBRARY_PATH=/tmp/lib:LD_LIBRARY_PATH
-- export PATH=/tmp/bin:$PATH
-- git clone --depth 1 https://github.com/jedisct1/libsodium.git --branch=stable
-- cd libsodium
-- env ./configure --disable-dependency-tracking --prefix=/tmp
-- make -j$(nproc) install
-- cd -
 - git clone --depth 1 https://github.com/jedisct1/minisign.git
 - cd minisign/src
 - gcc -O2 -o /tmp/bin/minisign -I/tmp/include -L/tmp/lib *.c -lsodium