#253 Replace removed Travis install with no-op

Per [1] Travis will do some autoamtic setup steps
based on the kind of build, which for `python` builds
seems to involve automatically installing our
_requirements.txt_ file prior to running the Docker
build process we actually want.

See for example the `pip install -r requirements.txt`
command executed in this build [2] that takes almost
100 seconds.

This change overrides Travis's `install` step with
a no-op directive.

[1]: https://docs.travis-ci.com/user/customizing-the-build#Customizing-the-Installation-Step
[2]: https://travis-ci.org/ic-labs/django-icekit/builds/245610276

.travis.yml

@@ -5,6 +5,10 @@ language: python
 services:
   - docker
 
+# Skip any default but pointless (for us) work Travis does for `python` builds:
+# https://docs.travis-ci.com/user/customizing-the-build#Customizing-the-Installation-Step
+install: true
+
 before_script:
   - docker --version
   - docker-compose --version