option of uninstall error fix

* remove source without --purge option
* remove current version without --fouce option

current version is 9.3.2
```
~$ pgvm list
PostgreSQL Installed Version:

    9.1.11    
    9.2.6     
 => 9.3.2     
~$ pgvm current
9.3.2
~$ ls -al ~/.pgvm/src
total 126312
drwxr-xr-x  11 sasakimakoto  staff       374 12  9 05:41 .
drwxr-xr-x  14 sasakimakoto  staff       476 12  9 05:12 ..
drwxr-xr-x  23 sasakimakoto  staff       782 12  9 05:43 postgresql-9.1.11
-rw-r--r--   1 sasakimakoto  staff  20794895 12  9 05:41 postgresql-9.1.11.tar.gz
-rw-r--r--   1 sasakimakoto  staff        59 12  9 05:41 postgresql-9.1.11.tar.gz.md5
drwxr-xr-x  23 sasakimakoto  staff       782 12  9 05:41 postgresql-9.2.6
-rw-r--r--   1 sasakimakoto  staff  21641399 12  9 05:38 postgresql-9.2.6.tar.gz
-rw-r--r--   1 sasakimakoto  staff        58 12  9 05:38 postgresql-9.2.6.tar.gz.md5
drwxr-xr-x  23 sasakimakoto  staff       782 12  9 05:37 postgresql-9.3.2
-rw-r--r--   1 sasakimakoto  staff  22218672 12  9 05:35 postgresql-9.3.2.tar.gz
-rw-r--r--   1 sasakimakoto  staff        58 12  9 05:35 postgresql-9.3.2.tar.gz.md5
~$
```

pgvm uninstall 9.3.2
```
~$ pgvm uninstall 9.3.2
Removing Postgres version 9.3.2 installed and your source by pgvm!

~$ pgvm list
PostgreSQL Installed Version:

    9.1.11    
    9.2.6     
~$ ls -al ~/.pgvm/src
total 82904
drwxr-xr-x   8 sasakimakoto  staff       272 12  9 05:48 .
drwxr-xr-x  14 sasakimakoto  staff       476 12  9 05:47 ..
drwxr-xr-x  23 sasakimakoto  staff       782 12  9 05:43 postgresql-9.1.11
-rw-r--r--   1 sasakimakoto  staff  20794895 12  9 05:41 postgresql-9.1.11.tar.gz
-rw-r--r--   1 sasakimakoto  staff        59 12  9 05:41 postgresql-9.1.11.tar.gz.md5
drwxr-xr-x  23 sasakimakoto  staff       782 12  9 05:41 postgresql-9.2.6
-rw-r--r--   1 sasakimakoto  staff  21641399 12  9 05:38 postgresql-9.2.6.tar.gz
-rw-r--r--   1 sasakimakoto  staff        58 12  9 05:38 postgresql-9.2.6.tar.gz.md5
~$
```