Make compatible with Rails 4.1 and Rails 4.2

:bullettrain_side::arrow_upper_right: 

### Description
 - Add 4.1 and 4.2 gemfiles and configure Travis
 - Add actionview to 4.2 gemfile because it's not autoloaded by actionpack
 - Fix testhelper to be compatible with minitest 5 and rails >= 4.1
 - Fix spec for 4.2 because Symbol is casted into String

### Steps to merge
 - [ ] :+1: of the team
:+1: from me ... afaik a some ppl want to have it that way ...

On Tue, Oct 28, 2014 at 2:15 PM, Benjamin Quorning <notifications@github.com
> wrote:

> In .gitignore:
>
> > @@ -7,4 +7,4 @@ doc
> >  .yardoc
> >  .bundle
> >  pkg
> > -gemfiles/*.lock
> > +gemfiles/*.gemfile.lock
>
> Shouldn’t we remove this line and actually add those lockfiles to the
> repository?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/zendesk/property_sets/pull/45/files#r19504417>.
>