Travis CI, unable to run tests - Error: activesupport requires Ruby version >= 2.2.2

I have an open sourced Obj-C repository with Kiwi unit testing framework as CocoaPods dependency.
To run tests I'm using Travis CI continious integration tool.

Everything was fine previously, all unit tests were executed, but now I'm getting Travis CI build error:

ERROR:  Error installing cocoapods:
    activesupport requires Ruby version >= 2.2.2.

Here is my .travis.yml file:

language: objective-c

before_install:
- sw_vers
- cd EKAlgorithmsApp

install: make install

script: make ci

And here is my Makefile:

install:
    gem install cocoapods --no-ri --no-rdoc
    gem install xcpretty  --no-ri --no-rdoc
    pod install

ci:
    xcodebuild -workspace EKAlgorithmsApp.xcworkspace/ -scheme EKAlgorithmsSpecs -sdk macosx -configuration Release clean test | xcpretty -c

It seems that something in configurations has changed on Travis CI side, but I'm not very familiar with Ruby, so I need a help of how to fix that warning to run my tests successfully.

Solved it by adding next line to my .travis.yml file:

osx_image: xcode7.3

So my .yml file now looks like:

language: objective-c
osx_image: xcode7.3

before_install:
- sw_vers
- cd EKAlgorithmsApp

install: make install

script: make ci