.travis.yml: Get schema dependencies in before_install

Avoid:

  $ make .govet
  go vet -x ./...
  schema/validate.go:10:2: cannot find package "github.com/xeipuuv/gojsonschema" in any of:
  	/home/travis/.gimme/versions/go1.10.1.linux.amd64/src/github.com/xeipuuv/gojsonschema (from $GOROOT)
  	/home/travis/gopath/src/github.com/xeipuuv/gojsonschema (from $GOPATH)
  make: *** [.govet] Error 1
  The command "make .govet" exited with 2.

with Go 1.10.1 [1].  Go 1.9.5 doesn't seem to mind [2], but it
shouldn't mind the presence of the dependencies either.

[1]: https://travis-ci.org/opencontainers/runtime-spec/jobs/376991027#L531
[2]: https://travis-ci.org/opencontainers/runtime-spec/jobs/376991028#L529

Signed-off-by: W. Trevor King <wking@tremily.us>

Author: wking

.travis.yml

@@ -11,6 +11,7 @@ services:
 before_install:
   - make install.tools
   - docker pull vbatts/pandoc
+  - go get -d ./schema/...
 
 install: true