Update travis.yml

[joachifm]

• Test against libsodium 1.0.2 and add libsodium version to the build matrix
• Test against GHC 7.8.4, 7.6.3, and 7.10, allowing the latter two to fail.

[tel]

That's pretty great. Thanks for the contribution!

If you are an active user of saltine and would like to consider commit bit rights/responsibilities please let me know and we can discuss.