Update Travis config to Python 3.7

.travis.yml

@@ -1,19 +1,6 @@
 sudo: false
 language: python
 python:
-  - "2.7"
-env:
-  - PIP_DOWNLOAD_CACHE="pip_cache"
-cache:
-  directories:
-    - pip_cache
-install:
-  - pip install -r requirements.txt
+  - "3.7"
+install: pip install -r requirements.txt
 script: python check_advisories.py --all
-notifications:
-  irc:
-    channels:
-      - "irc.mozilla.org#www"
-    on_success: never
-    on_failure: always
-    use_notice: true