travis, build, consensus: drop support for Go 1.7

[karalabe]

This PR drops support for building with Go 1.7. The reasons are:

• Starting with the release of Go 1.10, the last supported Go version for security fixes is 1.9 (not even 1.8) https://golang.org/doc/devel/release.html#policy . As such, it's actually advisable not to support old Go versions for us.
• Go 1.8 introduces certain testing helpers that @nonsense and the Swarm team rely on, so it's simpler to drop 1.7 than to keep patching around the missing features.
• Go 1.7 does not support detecting Bit Manipulation Instruction sets for the CPUs, so crypto libraries either need patches for 1.7, cannot use advanced CPU features, or will simply fail to build (e.g. cloudflare bn256 core/vm, crypto/bn256: switch over to cloudflare library #16203).

[nonsense]

LGTM