Require version file for checking capistrano version

bundler-1.4.0.pre.1 works fine, but bundler-1.4.0.pre.2 shows error below:
```
$ bundle exec cap production deploy
/path/to/bundler-1.4.0.pre.2/lib/bundler/capistrano.rb:7:in `<top (required)>': uninitialized constant Capistrano::Version (NameError)
```

This commit brings this error, so I add require version file.
1d7a7893dfeb85afa25a3a526354b31c85f05dbf

bundler: 1.4.0.pre.2
capistrano: 2.14.2
Guard from load error 'capistrano/version'
Simple check const exist
Merge pull request #2616 from sanemat/fix/if-capistrano

Require version file for checking capistrano version