Switch travis to bionic, remove trusty hacks

.travis.yml

@@ -1,6 +1,6 @@
 language: cpp
 sudo: required
-dist: trusty
+dist: bionic
 matrix:
   include:
     # Note that we stick some additional variations to some builds
@@ -15,10 +15,8 @@ matrix:
     - compiler: clang
       env: CXXSTD=c++1y BUILD_STATIC=yes
 before_install:
-  - sudo add-apt-repository --yes ppa:zoogie/sdl2-snapshots
   - sudo apt-get update -qq
   - sudo apt-get install -qq cmake libsdl2-dev cppcheck doxygen pulseaudio
-  - sudo sed -i -e '/using ::gets/ d' /usr/include/c++/4.8/cstdio # build failure with clang/c++1y
   - |-
     if [ -n "${BUILD_COVERAGE}" ]; then
         export CMAKE_EXTRA_ARGS="${CMAKE_EXTRA_ARGS} -DCMAKE_BUILD_TYPE=Coverage"