tests: move to using tox 1.7.5

When we moved to having tox run on tests/ and tools/ we bumped tox
to version 1.7.1.  That was an error on my part as I just checked the
version I had locally and didn't go looking for what the newest upstream
release was.

The current version as of today is 1.7.5 and 'tox -e pylint' works with
this version

1 files changed, 1 insertions, 1 deletions

diff --git a/tox.ini b/tox.ini
index d7316cc2..0802072a 100644
--- a/tox.ini
+++ b/tox.ini
@@ -24,7 +24,7 @@ setenv =
 basepython = python3
 deps =
     # requirements
-    pylint==1.7.1
+    pylint==1.7.5
     # test-requirements because unit tests are now present in cloudinit tree
     -r{toxinidir}/test-requirements.txt
 commands = {envpython} -m pylint {posargs:cloudinit tests tools}