Use from gem

Use tachikoma from gem

```
/Gemfile
source "https://rubygems.org"
gem 'tachikoma', github: 'sanemat/tachikoma', branch: 'feature/gemize'
```

```
/.gitignore
/repos/*
!/repos/.gitkeep
```

```
/Rakefile
require 'bundler/setup'
require 'tachikoma/tasks'
```

```
$ mkdir -p data
$ mkdir -p repos
$ touch repos/.gitkeep
# Add your yaml file
```

Then, `BUILD_FOR=foo TAKEN_FOO=foo bundle exec tachikoma:load tachikoma:fetch tachikoma:bundle tachikoma:pull_request`