Travis-CI: add go_import_path
Travis-CI: add go_import_path

The go_import_path option of Travis-CI allows to tell Travis-CI the path where
the code should be cloned in $GOPATH.
This allows external contributors to use Travis-CI on their fork without
incorrectly running the testsuite with the original import path instead of their
modified code.

dolmen-go/jsonptrerror