Update travis to use more recent python and pytest versions

- Fix current "python" environment on travis to 3.5
- Updated build matrix to give preference of testing all pytest versions in py27 and py35

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist=
-  py{26,33,34,27}-pytest2{4,5,6,7},py{27,34}-pytest27-pexpect,flakes,readme
+  py{26,27,34,35}-pytest2{4,5,6,7,8},py{27,34}-pytest28-pexpect,flakes,readme
 
 
 [testenv]
@@ -10,9 +10,9 @@ deps =
     setuptools_scm # to avoid .eggs
     pytest24: pytest~=2.4.0
     pytest25: pytest~=2.5.0
-
     pytest26: pytest~=2.6.1
     pytest27: pytest~=2.7.2
+    pytest28: pytest~=2.8.3
     pexpect: pexpect
 commands=
   # always clean to avoid code unmarshal mismatch on old python/pytest