Fix "selfupdate" and Use git itself to check if ANTIGEN_INSTALL_DIR is a git repository.

```
»  zsh --version                                                          
zsh 5.0.5 (i686-pc-linux-gnu)
```
* I installed antigen using ```git clone```. When running the ```antigen selfupdate``` command, it gives this error
```
Your copy of antigen doesn't appear to be a git clone. 
The 'selfupdate' command cannot work in this case.
```
As ```_ANTIGEN_INSTALL_DIR``` will be used later by ```antigen-selfupdate```, it should be exported. Or ```cd $_ANTIGEN_INSTALL_DIR``` within
```
antigen-selfupdate () {
    ( cd $_ANTIGEN_INSTALL_DIR
      # ...
    )
} 
```
will navigate to the home directory.

* I think it's better to use git itself to check if _ANTIGEN_INSTALL_DIR is a git repository. This can be done via
```
git rev-parse &> /dev/null
if [[ $? -ne 0 ]]; then
   echo "Your copy of antigen doesn't appear to be a git clone. " \
           "The 'selfupdate' command cannot work in this case."
   return 1
fi
```