Docker command provided in readme fails

Running

```
docker run --rm -v `pwd`:/go/src/in -v `pwd`:/out gopy/gopy app bind -output=/out in
```

from the root of the repository results in this error:

```
panic: could not look-up "interface{}"!

goroutine 1 [running]:
github.com/go-python/gopy/bind.(*symtab).addSliceType(0xc420160360, 0xc420011630, 0x838ac0, 0xc420011810, 0x834540, 0xc420014ff0, 0x8, 0xc4201604a0, 0x1f, 0xc4200156a0, ...)
/go/src/github.com/go-python/gopy/bind/symtab.go:552 +0x9fc
github.com/go-python/gopy/bind.(*symtab).addType(0xc420160360, 0x838ac0, 0xc420011810, 0x834540, 0xc420014ff0)
/go/src/github.com/go-python/gopy/bind/symtab.go:389 +0x1213
github.com/go-python/gopy/bind.(*symtab).processTuple(0xc420160360, 0xc42000b140)
/go/src/github.com/go-python/gopy/bind/symtab.go:357 +0xe3
github.com/go-python/gopy/bind.(*symtab).addSymbol(0xc420160360, 0x8387a0, 0xc420011860)
/go/src/github.com/go-python/gopy/bind/symtab.go:337 +0xb73
github.com/go-python/gopy/bind.(*Package).process(0xc42009a960, 0x0, 0xc42006bad0)
/go/src/github.com/go-python/gopy/bind/package.go:182 +0x1f2c
github.com/go-python/gopy/bind.NewPackage(0xc420011630, 0xc4200b9960, 0x0, 0x0, 0x0)
/go/src/github.com/go-python/gopy/bind/package.go:47 +0x2c6
main.newPackageFrom(0xc420088a80, 0xc420011630, 0x2, 0xc420011630, 0x0)
/go/src/app/gen.go:222 +0xeb
main.newPackage(0x7ffd3612ef4c, 0x2, 0xc42001491c, 0x4, 0x0)
/go/src/app/gen.go:199 +0x5b5
main.gopyRunCmdBind(0xc4200ae6c0, 0xc42000e130, 0x1, 0x1, 0x0, 0x0)
/go/src/app/cmd_bind.go:82 +0x3b9
github.com/gonuts/commander.(*Command).Dispatch(0xc4200ae6c0, 0xc42000e120, 0x2, 0x2, 0x1, 0x7f544dab5000)
/go/src/github.com/gonuts/commander/commands.go:221 +0x20a
github.com/gonuts/commander.(*Command).Dispatch(0xc4200ae7e0, 0xc42000e110, 0x3, 0x3, 0x0, 0x0)
/go/src/github.com/gonuts/commander/commands.go:187 +0x6a4
main.run(0xc42000e110, 0x3, 0x3, 0xc42003bf78, 0x671709)
/go/src/app/main.go:35 +0x297
main.main()
/go/src/app/main.go:43 +0x6c
```

Also, I'm. not sure if it's related but `bind` is not currently part of the CLI but it's used in the command sent to Docker. Looking at historical issues `bind` seems to be part of an older backwards-incompatible version of the CLI