Change way of running travis tests

In order to properly run tests for Rails 4.1 we need to have separate gemfiles
for MongoMapper and Mongoid (conflict of gem versions). In order to handle this
  properly I decided to change the way we run tests. This change will allow to
  later add separate Gemfiles for Rails 4.1
exclude datamapper tests with rails4 ruby 2.1
Merge pull request #544 from arnvald/improve_travis_matrix

Change way of running travis tests