Update Travis CI configuration

Previously, the configuration for Travis CI was very busy and contained
a lot of things that we were not sure about, which was causing the build
to be unnecessarily slow. The configuration has been updated to match
that in Thoughtbot's suspenders repository.

https://trello.com/c/HfxmDKJF
Update sample environment variables

Previously, some sensitive data was being stored in the application's
sample environment variables, which was presenting a security issue. The
variables have been updated to use dummy values.

https://trello.com/c/ss5habYR