Update the linear gradient rule

`linear-gradient` with value of `0deg`, starts from the bottom to the top. To achieve the same effect as the vendor prefixed version of the gradients, the value should be `180deg`
test rails 4.2 on travis
Merge pull request #3708 from mupkoo/patch-1

Update the linear gradient rule
add rails 4.2 node
fix grammar in README
set Rails 4.2 as an allowed failure on Travis
fix travis allow_failures
Merge pull request #3709 from activeadmin/travistest

fix travis allow_failures
fix typo in columns.rb
test against Ruby 2.2

& don't needlessly specify the tiny version number
allow failures for 2.2 and keep testing on 2.1 until 2.2 tests are passing
Merge pull request #3714 from activeadmin/testtravis

allow failures for 2.2 and keep testing on 2.1 until 2.2 tests are passing
add `fast_finish: true` to travis.yml
change travis matrix
Relaxed rails version restriction for Rails 4.2 support.