Explicitly fetch `goimports`

Fetching the whole tree was failing on some Go versions and we really
only need goimports.

.travis.yml

@@ -26,7 +26,7 @@ matrix:
 
 before_script:
 - go get github.com/urfave/gfmrun/... || true
-- go get golang.org/x/tools/... || true
+- go get golang.org/x/tools/cmd/goimports
 - if [ ! -f node_modules/.bin/markdown-toc ] ; then
     npm install markdown-toc ;
   fi