travis CI: revert to Go 1.7

Because gopherjs master branch still needs Go 1.7.
So we either need to include a Go 1.7 with CAMLI_GOPHERJS_GOROOT in the
travis build (which I will look into later), or wait for gopherjs to
support Go 1.8.

Change-Id: I8e991de279bf822e1b5ef64dcae241880fe3eb73

.travis.yml

@@ -3,7 +3,7 @@ dist: trusty
 go_import_path: camlistore.org
 language: go
 go:
-  - 1.8
+  - 1.7
 before_install:
   - sudo apt-get install -qq pkg-config fuse
   - sudo modprobe fuse