Make Travis-CI use the canonical import path

Currently we get an import problem for forks, for example
“someone-else/harmony”.  Travis-CI job set up for such a fork will check
out the repo under GOPATH/src/github.com/someone-else/harmony,
importable as "github.com/someone-else/harmony" and not as the intended
"github.com/harmony-one/harmony".

Recording the canonical import path (go_import_path) in .travis.yml lets
Travis-CI check out the repo under GOPATH/src always using the canonical
import path, resolving build issues for forks.

.travis.yml

@@ -1,6 +1,7 @@
 language: go
 go:
   - stable
+go_import_path: github.com/harmony-one/harmony
 install:
   - export GOPATH=$HOME/gopath
   - export CGO_CPPFLAGS="-I$GOPATH/src/github.com/harmony-one/bls/include -I$GOPATH/src/github.com/harmony-one/mcl/include"