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

.travis.yml

@@ -1,7 +1,7 @@
 sudo: false
 language: python
 python:
-    - '3.5.0b3'
+    - '3.5'
 # command to install dependencies
 install: "pip install -U tox setuptools_scm"
 # # command to run tests
@@ -10,23 +10,30 @@ env:
   - TESTENV=flakes
   - TESTENV=readme
   # matrix was trimmed to skip
-  # some builds that are unnecessary/perceived redundant
-  - TESTENV=py26-pytest24
-  - TESTENV=py26-pytest25
-  - TESTENV=py26-pytest26
+  # some builds that are unnecessary/perceived redundant:
+  # py27 and py35 are tested with all pytest versions we declare to support;
+  # py26 and py34 only with latest pytest versions.
   - TESTENV=py26-pytest27
-  - TESTENV=py33-pytest27
-  - TESTENV=py34-pytest24
-  - TESTENV=py34-pytest25
-  - TESTENV=py34-pytest26
+  - TESTENV=py26-pytest28
+
   - TESTENV=py27-pytest24
   - TESTENV=py27-pytest25
   - TESTENV=py27-pytest26
+  - TESTENV=py27-pytest27
+  - TESTENV=py27-pytest28
 
-  - TESTENV=py27-pytest27-pexpect
-  - TESTENV=py34-pytest27-pexpect
-#  - TESTENV=py35-pytest27
-  - TESTENV=pypy-pytest27
+  - TESTENV=py34-pytest27
+  - TESTENV=py34-pytest28
+
+  - TESTENV=py35-pytest24
+  - TESTENV=py35-pytest25
+  - TESTENV=py35-pytest26
+  - TESTENV=py35-pytest27
+  - TESTENV=py35-pytest28
+
+  - TESTENV=py27-pytest28-pexpect
+  - TESTENV=py34-pytest28-pexpect
+  - TESTENV=pypy-pytest28
 
 script: tox --recreate -e $TESTENV