Add pytest 2.9 and drop pytest 2.4 and 2.5 from build matrix

tox.ini

@@ -1,8 +1,8 @@
 [tox]
 # if you change the envlist, please update .travis.yml file as well
 envlist=
-  py{26,27,34}-pytest2{4,5,6,7,8}
-  py35-pytest2{7,8}
+  py{26,27,34}-pytest2{6,7,8,9}
+  py35-pytest2{7,8,9}
   py{27,35}-pytest28-pexpect
   flakes
   readme  
@@ -14,11 +14,10 @@ passenv = USER USERNAME
 deps =
   pycmd
   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
+  pytest28: pytest~=2.8.7
+  pytest29: pytest~=2.9.1
   pexpect: pexpect
 platform=
   pexpect: linux|darwin