Streamline the travis-ci config.

[otrho]

It's now passing again.

• Switched to just the latest versions of dmd and ldc.
• Still only building, no tests as they're currently broken.
• Still building OpenSSL as a dependency, though this could still be streamlined further (don't need to install all the docs, etc.).
• Removed travis.sh and moved travis_install_deps.sh to .travis-install-openssl.sh.
• Don't call the codecov.io script since our tests aren't running.