Refresh the CI

Signed-off-by: Vadim Markovtsev <vadim@sourced.tech>

.travis.yml

@@ -1,22 +1,15 @@
 language: python
 cache: pip
+dist: bionic
 linux_before_install: &linux_before_install
   - pip3 install --upgrade pip
 matrix:
   include:
     - language: generic
       os: osx
-    - python: 3.4
-      dist: trusty
-      before_install: *linux_before_install
-    - python: 3.5
-      dist: xenial
-      before_install: *linux_before_install
     - python: 3.6
-      dist: xenial
       before_install: *linux_before_install
     - python: 3.7
-      dist: xenial
       before_install: *linux_before_install
 install:
   - pip3 install numpy scipy pycodestyle codecov