Break solidus into components

Fixes the following error when running `bundle lock`:

```
Bundler could not find compatible versions for gem "solidus_backend":
  In Gemfile:
    solidus_backend (>= 2.6, < 4)

    solidus was resolved to 3.2.0.alpha, which depends on
      solidus_backend (= 3.2.0.alpha)

Could not find gem 'solidus_backend (= 3.2.0.alpha)', which is required by gem 'solidus', in rubygems
repository https://rubygems.org/ or installed locally.
```

Author: gsmendoza

Gemfile

@@ -4,7 +4,14 @@ source 'https://rubygems.org'
 git_source(:github) { |repo| "https://github.com/#{repo}.git" }
 
 branch = ENV.fetch('SOLIDUS_BRANCH', 'master')
-gem 'solidus', github: 'solidusio/solidus', branch: branch
+
+git "https://github.com/solidusio/solidus.git", branch: branch do
+  gem 'solidus_api'
+  gem 'solidus_backend'
+  gem 'solidus_core'
+  gem 'solidus_frontend'
+  gem 'solidus_sample'
+end
 
 gem 'rails', ENV.fetch('RAILS_VERSION', nil)