shell.js grep: internal error,  Invalid regular expression

### Node version: 6.1.0
### ShellJS version (the most recent version/Github branch you see the bug on): 0.7.0
### Operating system: Mac OS
### Description of the bug:

```
shell.js: internal error
SyntaxError: Invalid regular expression: /*.js/: Nothing to repeat
```
### Example ShellJS command to reproduce the error:

``` javascript
shelljs.ls().grep("*.js")
```
