Set not to look "system" gem directories when install bundler to poxen/.gems

## Problem

Run `script/poxen` in my environment:

```
$ ./script/poxen
./script/poxen: line 22: /Users/gongo/poxen/.gems/bin/bundle: No such file or directory
./script/poxen: line 23: /Users/gongo/poxen/.gems/bin/bundle: No such file or directory
```

## Cause

1. `bundler` was already been installed in my `system` environment (not poxen direcotory)

    ```
    $ /usr/bin/gem list -i bundler -v "~> 1.3"
    true
    $ /usr/bin/gem env
      (..skip)
      - GEM PATHS:
         - /Library/Ruby/Gems/2.0.0
         - /Users/gongo/.gem/ruby/2.0.0
         - /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/gems/2.0.0
      (..skip)
    ```

2. When run `scirpt/poxen`, skip `bundler` installation process.

    ```sh
    (/usr/bin/gem list -i bundler -v "~> 1.3" > /dev/null) || {
      /usr/bin/gem install --no-rdoc --no-ri bundler   # Not run
    }
    ```

3. `poxen/.gems` is left empty because `GEM_HOME=poxen/.gems gem install bundler` not run, 
4. This problem occurs.

## Solution

Set `GEM_PATH` when install bundler to `poxen/.gems`:

```
$ GEM_HOME=/path/to/poxen/.gems GEM_PATH=$GEM_HOME /usr/bin/gem env
(..skip)
  - GEM PATHS:
     - /path/to/poxen/.gems
(..skip)

$ GEM_HOME=/path/to/poxen/.gems /usr/bin/gem list -i bundler -v "~> 1.3"
true

$ GEM_HOME=/path/to/poxen/.gems GEM_PATH=$GEM_HOME /usr/bin/gem list -i bundler -v "~> 1.3"
false
```

Only when `bundler` is not installed on `poxen/.gems`, run `gem install bundler` to `poxen/.gems`.
Thank you for sending a pull request! I see your point and it makes sense to me.
However, instead of giving `GEM_PATH` here, export `GEM_PATH` environment variable set as GEM_HOME for entire `script/poxen` script must be better to isolate poxen gem environment.