Update activerecord/activesupport constraints to allow Rails 6

[zokioki]

Rails 6 is currently in beta2 and some large shops are already running it in production; this PR updates the dependency constraints to allow for Rails 6.

[coveralls]

Coverage remained the same at 100.0% when pulling 94a1ccb on zokioki:rails-6 into 22df9b0 on felipediesel:master.

[coveralls]

Coverage decreased (-42.1%) to 57.895% when pulling 75ed3cd on zokioki:rails-6 into 22df9b0 on felipediesel:master.

[coveralls]

Coverage decreased (-42.1%) to 57.895% when pulling 75ed3cd on zokioki:rails-6 into 22df9b0 on felipediesel:master.

[coveralls]

Coverage decreased (-42.1%) to 57.895% when pulling 75ed3cd on zokioki:rails-6 into 22df9b0 on felipediesel:master.

[coveralls]

Coverage decreased (-42.1%) to 57.895% when pulling 75ed3cd on zokioki:rails-6 into 22df9b0 on felipediesel:master.

[coveralls]

Coverage decreased (-42.1%) to 57.895% when pulling 75ed3cd on zokioki:rails-6 into 22df9b0 on felipediesel:master.

[coveralls]

Coverage decreased (-42.1%) to 57.895% when pulling 75ed3cd on zokioki:rails-6 into 22df9b0 on felipediesel:master.

[felipediesel]

@zokioki Thanks for the PR. But it's weird that the coverage has decreased.

[zokioki]

@felipediesel That seems to be only for the first commit (where sqlite3 failed to install as a dev dependency) - after 94a1ccb the coverage went back up to 100%, though coveralls didn't seem to post a comment to the PR on it.

See https://coveralls.io/builds/22098724

[felipediesel]

@zokioki I realize that your changes are in fact not allowing rails 6. Only version smaller than 6. So I'm closing this PR and I have a branch ready to be merged to master. I'm also upgrading the versions tested and setting the Ruby version.

[zokioki]

@felipediesel Ah yes, I should have set that to read 6.1, good catch. Thanks for merging your changes in!