Add Go 1.12, 1.13 to build matrix; drop 1.9, 1.10
Add Go 1.12, 1.13 to build matrix; drop 1.9, 1.10

Adds newer versions of Go 1.12 and 1.13 to the Travis build matrix.

We also drop 1.9 and 1.10 -- unfortunately, go-redis has gone entirely
in on Go modules and doesn't work without them anymore. They were first
introduced in Go 1.11, so only our builds in 1.11+ are working.

There's probably a workaround we could use to keep the old versions
building, but it's not worth it. Throttled should still work fine on
these older versions even if they're not in the matrix, and Go users
tend to be pretty good about keeping their Go up-to-date anyway, so I
doubt we'll be affecting many people.

PuerkitoBio/throttled,throttled/throttled,throttled/throttled