Fix travis

[jsoo1]

• Move to xenial from trusty per stack travis docs
• Don't use x-toolkit in batch mode
• Remove already-installed libraries
• No jpeg in batch mode
• No gnutls for tests
• Don't use ffi or gmp
• test 24.5, remove 24.3 and 24.4 since debian stretch uses 24.5

[david-christiansen]

Thanks so much for taking care of this!

[jsoo1]

My pleasure! Thanks for all your contributions, here and elsewhere. 😃