Remove tox workaround for Travis CI

No longer necessary.

.travis.yml

@@ -19,11 +19,8 @@ matrix:
     - python: 3.6
       env: TOXENV=py36
     - python: pypy
-      env: TOXENV=pypy PYPY_VERSION=5.1
+      env: TOXENV=pypy
 
-install:
-    - ./.travis/install.sh
+install: pip install tox
 
-script:
-  - source ~/.venv/bin/activate
-  - tox
+script: tox

.travis/install.sh

@@ -1,17 +0,0 @@
-# temporary pyenv installation to get latest pypy until the travis
-# container infra is upgraded
-if [[ "${TOXENV}" = pypy* ]]; then
-    git clone https://github.com/yyuu/pyenv.git ~/.pyenv
-    PYENV_ROOT="$HOME/.pyenv"
-    PATH="$PYENV_ROOT/bin:$PATH"
-    eval "$(pyenv init -)"
-    pyenv install "pypy-$PYPY_VERSION"
-    pyenv global "pypy-$PYPY_VERSION"
-
-    pyenv rehash
-fi
-
-pip install --upgrade virtualenv
-virtualenv ~/.venv
-source ~/.venv/bin/activate
-pip install --upgrade tox