CI: Pin Travis golang version

Travis appears to be providing a version of golang that is too old for
https://mvdan.cc/xurls/cmd/xurls, which is used by the CI scripts in the
tests repo.

See:

- kata-containers/runtime#744
- kata-containers/tests#843 (comment)

Fixes kata-containers#281.

Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>

.travis.yml

@@ -7,7 +7,10 @@
 sudo: required
 dist: trusty
 
-language: bash
+language: go
+
+go:
+  - "1.10.x"
 
 before_script:
   - ".ci/setup.sh"