Update README.md install section.

This commit adds an additional step to the README.md install section to
run the go version command and check the version so people that are
installing it for the first time and ensure they are running a high
enough version.

README.md

@@ -48,10 +48,16 @@ https://github.com/btcsuite/btcd/releases
 - Install Go according to the installation instructions here:
   http://golang.org/doc/install
 
+- Ensure Go was installed properly and is the correct version:
+
+```bash
+$ go version
+```
+
 - Run the following command to obtain btcd, all dependencies, and install it:
 
 ```bash
-$ go get github.com/btcsuite/btcd/...
+$ go get -u github.com/btcsuite/btcd/...
 ```
 
 - btcd (and utilities) will now be installed in either ```$GOROOT/bin``` or