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.

.travis.yml

@@ -6,7 +6,9 @@ dist: trusty
 sudo: false
 
 go:
-    - 1.7  # Debian stable.
+    # Normally we'd want to support the version in Debian stable, but
+    # golang.org/x/crypto requires math/bits which appeared in 1.9.
+    - 1.9
     - stable
     - master