Remove .testrepository/times.dbm each test run

This overcomes a long standing bug in the testr tool: it doesn't
specify a dbm type to use, using a default which changes from
one python version to another (depending on build settings). It
is also unable to identify what was already used (which would be a
normal thing to do) and use that. Instead it just fails awkwardly.

The workaround is to remove the times.db file before each run
because it is not relevant.

tox.ini

@@ -6,8 +6,11 @@ envlist = py27,py33,py34,py35,py36,pypy,pep8,limit,failskip,docs,py35-prefix,py3
 [testenv]
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
+whitelist_externals = rm
 install_command = pip install -U {opts} {packages}
-commands = python setup.py testr --testr-args="{posargs}"
+commands =
+    rm -f .testrepository/times.dbm
+    python setup.py testr --testr-args="{posargs}"
 setenv = GABBI_PREFIX=
 passenv = GABBI_* HOME