⚗️ try to fix SSL issue

.travis.yml

@@ -95,13 +95,12 @@ matrix:
 
   - os: linux
     compiler: gcc
+    dist: bionic
     addons:
       apt:
         sources: ['ubuntu-toolchain-r-test']
         packages: ['g++-4.9', 'ninja-build']
     before_script:
-      - pip install --upgrade pip
-      - pip install --user urllib3[secure]
       - pip install --user cpp-coveralls
     after_success:
       - coveralls --build-root test --include include/nlohmann --gcov 'gcov-4.9' --gcov-options '\-lp'