Add `Gondler::Gomfile::NotFound`

In this pull req, I added `Gondler::Gomfile::NotFound` which is raised when there is no Gomfile.
If running `gondler` command without Gomfile, now `gondler` says a not found error (excepting using `help` subcommand).

```
➜ bundle exec bin/gondler
Gondler::Gomfile::NotFound
➜ bundle exec bin/gondler -h
Commands:
  gondler build           # Build with dependencies specified in your Gomfile
  gondler exec            # Execute a command in the context of Gondler
  gondler help [COMMAND]  # Describe available commands or one specific command
  gondler install         # Install the dependecies specified in your Gomfile
  gondler list            # Show all of the dependencies in the current bundle
  gondler repl            # REPL in the context of Gondler
  gondler test            # Test with dependencies specified in your Gomfile
  gondler version         # Print Gondler version

Options:
  [--gomfile=GOMFILE]  
                       # Default: Gomfile
  [--path=PATH]        
                       # Default: .gondler
```

(Plus I added `rake spec` task.)