Add instructions necessary for bin/test to work

Without these additional steps, `./bin/test` yields the following:

```
 Formatting packages...

 Installing package dependencies...

 Testing packages:
ok  	cf	0.904s
ok  	cf/api	7.987s
ok  	cf/app	0.824s
ok  	cf/commands	1.196s
ok  	cf/commands/application	1.299s
ok  	cf/commands/buildpack	0.828s
ok  	cf/commands/domain	0.757s
ok  	cf/commands/organization	0.598s
ok  	cf/commands/route	0.510s
ok  	cf/commands/service	0.634s
ok  	cf/commands/serviceauthtoken	0.447s
ok  	cf/commands/servicebroker	0.297s
ok  	cf/commands/space	0.911s
ok  	cf/commands/user	0.744s
ok  	cf/configuration	0.509s
ok  	cf/formatters	0.053s
ok  	cf/manifest	0.088s
ok  	cf/models	0.019s
ok  	cf/net	0.713s
ok  	cf/requirements	0.081s
ok  	cf/terminal	0.061s
ok  	cf/trace	0.046s
ok  	generic	0.044s

 Vetting packages for potential issues...
go tool: no such tool "vet"; to install:
	go get code.google.com/p/go.tools/cmd/vet

SUITE FAILURE
```

Mercurial must be installed, or else `go get code.google.com/p/go.tools/cmd/vet` yields:

```
go: missing Mercurial command. See http://golang.org/s/gogetcmd
package code.google.com/p/go.tools/cmd/vet: exec: "hg": executable file not found in $PATH
```