Change travis to only get Go dependencies for the current directory.

Previously it would fetch all the demo deps, which only work on Go 1.9 or later.

.travis.yml

@@ -3,14 +3,21 @@ sudo: false
 
 language: go
 go:
-  - 1.7.x
-  - 1.8.x
-  - 1.9.x
-  - tip
+  - "1.7.x"
+  - "1.8.x"
+  - "1.9.x"
+  - "1.10.x"
+  - "tip"
 
 before_install:
   - go get github.com/mattn/goveralls
   - go get golang.org/x/tools/cmd/cover
 
+# Only install and run the tests for the current directory
+# This avoids testing the demo directory (which has no tests) and
+# depends on packages that only support Go 1.9+.
+install:
+  - go get -t -v .
+
 script:
-  - travis_retry $HOME/gopath/bin/goveralls -service=travis-ci
+  - travis_retry $HOME/gopath/bin/goveralls -service=travis-ci -package .