Update travis config to use python 3.6+, and to install proper versions of dependencies, and to use bionic instead of trusty

.travis.yml

@@ -1,16 +1,15 @@
 language: python
 # sudo: required
-dist: trusty
+dist: bionic
 python:
-  - "3.4"
-  - "3.5"
   - "3.6"
   - "3.6-dev"
   - "3.7-dev"
   - "nightly"
 # command to install dependencies
 install:
-  - pip install Flask==0.12 stem==1.5.4 pytest-cov coveralls flake8
+  - pip install -r install/requirements.txt
+  - pip install pytest-cov coveralls flake8
 before_script:
     # stop the build if there are Python syntax errors or undefined names
     - flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics