travis: upgrade to bionic

This means we'll start enforcing no "maybe uninitialized" warnings at
-O3, since xenial was using gcc 5.4 or gcc 4.8 which are too primitive.

Seems like `sudo: false` is deprecated (those deps weren't being
installed); you simply install in the `before_install` hook.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

.travis.yml

@@ -1,20 +1,11 @@
 language: c
-dist: xenial
-sudo: false
+dist: bionic
 
 notifications:
   email: false
 
-addons:
-  apt:
-    packages:
-      - python3-pip
-      - libsqlite3-dev
-      - shellcheck
-      - cppcheck
-      - valgrind
-      - libomp-dev
-      - gcc-4.8
+before_install:
+  sudo apt-get install -y libsqlite3-dev cppcheck valgrind gcc-4.8
 
 env:
   - ARCH=64 SOURCE_CHECK_ONLY=true COPTFLAGS="-O3"

.travis/build.sh

@@ -27,7 +27,7 @@ if [ ! -f dependencies/bin/bitcoind ]; then
     rm -rf bitcoin-0.17.1-x86_64-linux-gnu.tar.gz bitcoin-0.17.1
 fi
 
-pyenv global 3.7
+pyenv global 3.7.1
 pip3 install --user --quiet mako
 pip3 install --user --quiet -r tests/requirements.txt
 pip3 install --quiet \