Drop Python 3.6 support in travis tests

Upgrade test cases from ubuntu xenial to bionic
Update README

.travis.yml

- language: python
-python:
-  - "3.6"
-  - "3.7"
-  - "3.8"
-  - "pypy3"
-# command to install dependencies
+language: python
+
+jobs:
+  include:
+    - name: "Python 3.7 with ubuntu 18.04 - Bionic"
+      os: linux
+      dist: bionic
+      python: 3.7
+    - name: "Python 3.8 with ubuntu 18.04 - Bionic"
+      os: linux
+      dist: bionic
+      python: 3.8
+    - name: "Python pypy3 with ubuntu 18.04 - Bionic"
+      os: linux
+      dist: bionic
+      python: pypy3
 install:
   - pip install python-dateutil
   - pip install pytz

README.md

-OPC UA / IEC 62541 Client and Server for Python >= 3.6 and pypy .
+OPC UA / IEC 62541 Client and Server for Python >= 3.7 and pypy3 .
 http://freeopcua.github.io/, https://github.com/FreeOpcUa/opcua-asyncio
 
 [![Build Status](https://travis-ci.org/FreeOpcUa/opcua-asyncio.svg?branch=master)](https://travis-ci.org/FreeOpcUa/opcua-asyncio)