build: update travis to build against Go 1.8 and 1.7.5

Sticking with our tradition of tracking the two latest go releases,
we'll now build against Go 1.8 (which was recently released and Go
1.7.5).

The release of Go 1.8 is very attractive to the project as it includes
performance and GC improvements as well as the addition of more
profiling and race condition detection capabilities within the runtime.

.travis.yml

@@ -1,7 +1,7 @@
 language: go
 go:
-  - 1.6.3
-  - 1.7.4
+  - 1.7.5
+  - 1.8
 sudo: false
 install:
   - go get -v github.com/roasbeef/btcd # Required for the rpc test package.