Test against multiple AS and Sinatra versions

This enables testing against Sinatra Edge and multiple activesupport versions. The change also
lowers the activesupport dependency to 3.1.

Failures in Sinatra Edge tests are ignored
Less matrix entries for sinatra_edge and ActiveSupport

Travis is free, so don't tax it too much with matrix entries that don't give much gain.
Merge remote-tracking branch 'origin/master' into multi-as-testing

Conflicts:
	.travis.yml
Merge pull request #1003 from padrino/multi-as-testing

Test against multiple AS and Sinatra versions