publishability via env vars is now determined without MAVEN_SETTINGS_ENV (2nd try)

When building pull requests, Travis doesn't set secret environment variable,
which means that even though MAVEN_SETTINGS_ENV was set,
SCALAREFLECT_MAVEN_USER and SCALAREFLECT_MAVEN_PASSWORD were not,
which led to a crash.

Therefore I did away with MAVEN_SETTINGS_ENV altogether and now trigger
loading Travis-style credentials by mere presence of all env variables.