Expand build matrix

[atz]

Currently testing Ruby 2.1 and Ruby 2.2.1 each with one 4.x version of rails, the same unreleased version of rsolr and essentially unspecified individual versions of Solr and Fedora. This is a pretty meagre reflection of the ecosystem the gem intends to service.

The range of dependencies includes:

• "activesupport", '>= 4.2.4', '< 6'
• 'rsolr', '>= 1.1.2', '< 3'
• required_ruby_version = '~> 2.0'

So either the dependencies should be tightened or the Travis build matrix should be expanded to include:

• Ruby: 2.0.x, 2.1.x, 2.2.x and JRuby (see #1101)
• activesupport: at least a representative of both major versions 4 and 5
• rsolr: at least a representative of both major versions 1 and 2

Arguably a couple different supported major versions of fedora and solr, also. I don't mind if individual matrix points are configured to allow_failure in the build at first, but I do care that we aren't even checking.

Not all versions will be compatible with each other (rails 5 on ruby 2.0, for example).