Upgrade supported Python version, drop 3.4

Python 3.4 is not supported anymore by 'lxml' which is
a PyEcore dependency. PyEcore only supports now Python > 3.5

.travis.yml

@@ -1,8 +1,9 @@
 language: python
 python:
-  - "3.4"
   - "3.5"
   - "3.6"
+  - "3.7"
+  - "3.8"
 install:
   - pip install --upgrade pip setuptools
   - pip install --upgrade pytest pytest-cov coveralls
@@ -18,4 +19,4 @@ deploy:
   distributions: "sdist bdist_wheel"
   on:
     tags: true
-    python: 3.6
+    python: 3.8

setup.py

@@ -4,8 +4,8 @@
 from setuptools import setup, find_packages
 from setuptools.command.test import test as TestCommand
 
-if sys.version_info < (3, 3):
-    sys.exit('Sorry, Python < 3.3 is not supported')
+if sys.version_info < (3, 5):
+    sys.exit('Sorry, Python < 3.5 is not supported')
 
 
 class PyTest(TestCommand):