Add -u to the README installation instructions.

This will allow the command to update the package if an old one happens to
be present while still fetching it if it's missing.

Suggested by @shurcooL in #28.

README.md

@@ -33,7 +33,7 @@ http://localhost:6060/pkg/github.com/davecgh/go-spew/spew
 ## Installation
 
 ```bash
-$ go get github.com/davecgh/go-spew/spew
+$ go get -u github.com/davecgh/go-spew/spew
 ```
 
 ## Quick Start