Add travis-lint to list of development dependencies

This ensures the gem is installed when we run our tests via Travis CI.

Change-Id: I2eef2593b89f94828a3cdf81854e305b8d13121a
Reviewed-on: http://gerrit.causes.com/45340
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>

Author: sds

overcommit.gemspec

@@ -30,4 +30,5 @@ Gem::Specification.new do |s|
 
   s.add_development_dependency 'image_optim', '~> 0.18.0'
   s.add_development_dependency 'rspec', '~> 3.0'
+  s.add_development_dependency 'travis-lint', '~> 2.0'
 end