Fixed build matrix for rails 3.2
Fixed build matrix for rails 4.x
Allowed failures for rails master in build matrix

Rails master is currently at 5.0.alpha and has unknown compatibility at
this point. Revisit later.
Dropped ruby 1.9.3 from build matrix

Ruby 1.9.3 is due for end-of-life soon anyway
Dropped ruby 2.0.0 from the build matrix

If you're on ruby 2.0, you should really be on 2.1 anyway.