Install tox in a virtualenv

Since TravisCI has an extremely old version of PyPy, we need to use
pyenv to install a more recent version. However, pyenv's shims don't
persist between the install and script portions of Travis. So we have to
use a virtualenv so Travis can find the tox command.

.travis.yml

@@ -25,4 +25,5 @@ install:
     - ./.travis/install.sh
 
 script:
+  - source ~/.venv/bin/activate
   - tox

.travis/install.sh

@@ -8,10 +8,10 @@ if [[ "${TOXENV}" = pypy* ]]; then
     pyenv install "pypy-$PYPY_VERSION"
     pyenv global "pypy-$PYPY_VERSION"
 
-    pip install --upgrade tox
-
     pyenv rehash
 fi
 
-
+python -m pip install --upgrade virtualenv
+python -m virtualenv ~/.venv
+source ~/.venv/bin/activate
 pip install --upgrade tox