Fix handlebars-source version dependency

Current master fe2c2deb couldn't resolve dependency using handlebars-source '1.0.0.rc3'.

For example:

```
# Gemfile
source 'https://rubygems.org'

gem 'barber', github: 'tchak/barber'
gem 'ember-source', '1.0.0.rc3'
gem 'handlebars-source', '1.0.0.rc3'
```

And

``` sh
$ bundle
```

In the result, the following error is got:

```
Bundler could not find compatible versions for gem "handlebars-source":
  In Gemfile:
    barber (>= 0) ruby depends on
      handlebars-source (>= 1.0.0.rc.4) ruby

    handlebars-source (1.0.0.rc3)
```

To fix this issue, I updated `barber.gemspec` to allow older version of handlebars-source.
(And I updated tests to prevent regression about this issue.)