Enable pylint on examples. (#51)

* Enable pylint on examples.

* Python3.7 enabled.

* updated dist and python versions.

* Minor fixes in travis script. Removed typo from pip install.

* Removed python3.8-dev from the list.

.travis.yml

@@ -1,9 +1,10 @@
 sudo : required 
+dist: xenial
 language: python
 python:
     - "2.7"
     - "3.6"
-    - "3.7-dev"
+    - "3.7"
 
 notifications:
     email:
@@ -16,11 +17,12 @@ before_script:
     - "export DISPLAY=:99.0"
     - "sh -e /etc/init.d/xvfb start"
     - sleep 2
-    - sudo apt install -yy python3-pip
+    - sudo apt install -yy python3-pip python3-setuptools python3-dev
 
 script:
     - # We use python3 to run the travis scripts. Which requires subprocess32 modules.
     - /usr/bin/python3 -m pip install subprocess32 --user
+    - /usr/bin/python3 -m pip install pylint --user
     - pip install pymoose --pre --no-deps
     - pip install pylint
     - pip install numpy --upgrade
@@ -30,3 +32,4 @@ script:
     - pip install libneuroml pyneuroml scipy matplotlib
     - pip install python-libsbml 
     - ./.travis/execute_travis_serial.py `which python`
+