This changes the travis-ci eigen download code to extract the tar on the
fly (rather than saving to a file first), and extracts into an `eigen`
directory rather than using upstream's `eigen-eigen-xxxxx` directory.

This also bumps the travis-ci eigen release to 3.3.4, in an attempt to
see if it fixed the -Wdeprecated warnings (it did not); the build setup
cleanup seems worth committing anyway.