remove the library path before we exec

In a fix to #148, I added unsetting these env vars into the Boot2Docker.app - but then reading #606 made me realise it should really be done in b2d-cli itself.

so 

Fixes #606 - maybe

This needs testing on a setup that has these env vars tho, as mine works all the time - i presume you need more dev tols installed locally?
`cmd.Env` is `nil` by default (in the object returned by `exec.Command()`), so this block won't ever actually do anything (http://golang.org/src/pkg/os/exec/exec.go?s=3459:3504#L102)

Why not just go the easy route and `os.Setenv("DYLD_LIBRARY_PATH", "")` etc?

Another solid option would be to just explicitly set `cmd.Env = []string{}`: (so that it's not set to the default of `nil`)
```go
        // Env specifies the environment of the process.
        // If Env is nil, Run uses the current process's environment.
        Env []string
```