Fixes version of ruby-saml to 0.6.* to avoid NameError when referencing OneLogin

We got this error when running bundle update in our project. This action updated ruby-saml (as required by omniauth-saml) to version 0.8.1. Our application then started throwing the following error:

```
A NameError occurred in #:

uninitialized constant OmniAuth::Strategies::SAML::Onelogin
/home/crm/production/source/shared/bundle/ruby/2.1.0/bundler/gems/omniauth-saml-840cc4706eaf/lib/omniauth/strategies/saml.rb:20:in `request_phase'
```

WIth the 0.6.* branch of ruby-saml this error does not occur, so I suggest that the version of ruby-saml should be fixed to 0.6.* until the real underlying issue is resolved.