Force pip<10 to install sdist package

Workaround for tox-dev/tox#467

Author: Marica Odagaki

.travis.yml

@@ -15,6 +15,6 @@ env:
 install:
   - "pip install tox tox-travis"
 script:
-  - "tox --force-dep pip$TEST_PIP_VERSION"
+  - "INSTALL_COMMAND_ARGS=${TEST_PIP_VERSION+--ignore-installed} tox -force-dep pip$TEST_PIP_VERSION"
 notifications:
   email: false

tox.ini

@@ -11,6 +11,7 @@ python =
     pypy: pypy
 
 [testenv]
+install_command = python -m pip install {env:INSTALL_COMMAND_ARGS:} {opts} {packages}
 commands = py.test --cov make_lambda_package {posargs}
 deps =
     pip