Use rails 5.0.0.beta3 in travis

[ianks]

No description provided.

[coveralls]

Coverage remained the same at 91.121% when pulling 99a324e on ianks:master into 401f43e on collectiveidea:master.

[albus522]

The ~> means new builds will automatically use the latest beta. Although this did run a new build to highlight rack is ruby >= 2.2.2 and mime-types-data doesn't work on jruby. So you could tweak the MRI part of the build matrix if you were looking for a contrib note, but the rails version tweak is not needed.