Remove custom repositories and cache dependencies during Travis CI build

[paulerickson]

The Travis build seems to take a ridiculously long time (more than an hour) downloading dependencies, before aborting due to lack of output.
I don't understand why a jar or pomfile should take minutes to download, but caching might help.

See https://docs.travis-ci.com/user/languages/groovy#Caching

[paulerickson]

Okay, it looks like it wasn't a caching issue (although it should help with build speed), but Travis must have looked in the non-public uw-releases repo and failed due to lack of credentials.

Removing the UW repositories, as well as the Apache snapshot repo, in favor of Maven Central should ensure the build is consistent and reproducible for anybody.

[timlevett]

Logical, if its open source, it should use open source repos.