tnvm.sh: fix egrep on MacOS (#30)
tnvm.sh: fix egrep on MacOS (#30)

**can't install node**
```
$ tnvm install node-v8.1.4
Version 'node-v8.1.4' not valid.
```
Tested on newly installed MacOS 10.12.5

`[-\.a-z]+?` does't match empty string
`([-\.a-z]+)?` does match empty string

```shell
 ~ echo 'node-abc' | egrep -o '[a-z]+-[a-z]+'
node-abc
~ echo 'node-' | egrep -o '[a-z]+-[a-z]+' 
~ echo 'node-' | egrep -o '[a-z]+-[a-z]+?' 
~ echo 'node-' | egrep -o '[a-z]+-([a-z]+)?'
node-
```
aliyun-node/tnvm,yjhjstz/tnvm,ali-sdk/tnvm