Use travis_retry in tox (via install_command), not on tox

This creates a wrapper script in bin/, exports `travis_retry` (a function)
into it and then injects this as `install_command` into tox.ini.

Closes pytest-dev#224

.travis.yml

@@ -45,5 +45,18 @@ matrix:
     - env: TESTENV=python3.4-master-sqlite
     - env: TESTENV=python3.4-master-sqlite_file
 install:
-  - travis_retry pip install tox
-script: travis_retry tox -e $TESTENV
+  # Create pip wrapper script, using travis_retry (a function) and
+  # inject it into tox.ini.
+  - mkdir -p bin
+  - PATH=$PWD/bin:$PATH
+  - printf '#!/bin/sh\n' > bin/travis_retry_pip
+  - declare -f travis_retry >> bin/travis_retry_pip
+  - printf '\necho "Using pip-wrapper.." >&2\ntravis_retry pip "$@"' >> bin/travis_retry_pip
+  - chmod +x bin/travis_retry_pip
+  - sed -i.bak 's/^\[testenv\]/\0\ninstall_command = travis_retry_pip install {opts} {packages}/' tox.ini
+  - diff tox.ini tox.ini.bak && false || true
+  - sed -i.bak 's/whitelist_externals =/\0\n    travis_retry_pip/' tox.ini
+  - diff tox.ini tox.ini.bak && false || true
+
+  - pip install tox
+script: tox -e $TESTENV

generate_configurations.py

@@ -227,8 +227,21 @@ def make_travis_yml(envs):
           allow_failures:
         %(allow_failures)s
         install:
-          - travis_retry pip install tox
-        script: travis_retry tox -e $TESTENV
+          # Create pip wrapper script, using travis_retry (a function) and
+          # inject it into tox.ini.
+          - mkdir -p bin
+          - PATH=$PWD/bin:$PATH
+          - printf '#!/bin/sh\\n' > bin/travis_retry_pip
+          - declare -f travis_retry >> bin/travis_retry_pip
+          - printf '\\necho "Using pip-wrapper.." >&2\\ntravis_retry pip "$@"' >> bin/travis_retry_pip
+          - chmod +x bin/travis_retry_pip
+          - sed -i.bak 's/^\[testenv\]/\\0\\ninstall_command = travis_retry_pip install {opts} {packages}/' tox.ini
+          - diff tox.ini tox.ini.bak && return 1 || true
+          - sed -i.bak 's/whitelist_externals =/\\0\\n    travis_retry_pip/' tox.ini
+          - diff tox.ini tox.ini.bak && return 1 || true
+
+          - pip install tox
+        script: tox -e $TESTENV
         """).strip("\n")
     testenvs = '\n'.join('  - TESTENV=%s' % testenv_name(env) for env in envs)
     checkenvs = '\n'.join('  - TESTENV=checkqa-%s' %