Fix travis build after change from "precise" to "trusty"

Travis recently has changed its dist from "precise" to "trusty", so
some Python versions are no longer installed by default

.travis.yml

@@ -11,12 +11,9 @@ env:
     - TOXENV=coveralls
     # note: please use "tox --listenvs" to populate the build matrix below
     - TOXENV=linting
-    - TOXENV=py26
     - TOXENV=py27
-    - TOXENV=py33
     - TOXENV=py34
     - TOXENV=py35
-    - TOXENV=pypy
     - TOXENV=py27-pexpect
     - TOXENV=py27-xdist
     - TOXENV=py27-trial
@@ -30,6 +27,12 @@ env:
 
 matrix:
   include:
+    - env: TOXENV=py26
+      python: '2.6'
+    - env: TOXENV=py33
+      python: '3.3'
+    - env: TOXENV=pypy
+      python: 'pypy-5.4'
     - env: TOXENV=py36
       python: '3.6'
     - env: TOXENV=py37