travis: Increase minimum supported version to 1.9
As of a29dc8fd (2019-04-26), golang.org/x/crypto requires math/bits. math/bits was introduced in Go 1.9, so we can no longer build from head using 1.7. Building using Debian's packaged dependencies should work just fine, but since we want Travis to build from head, this commit updates the minimum supported version to 1.9.
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