Update impressionist.gemspec

fix travis erros.

impressionist.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |s|
 
   s.files         = `git ls-files`.split('\n')
   s.test_files    = `git ls-files -- {spec,test_app/spec}/*`.split('\n')
-  s.require_paths = 'lib'
+  s.require_path = 'lib'
   s.required_rubygems_version = Gem::Requirement.new('>= 1.3.6') if s.respond_to? :required_rubygems_version=
 
   s.add_dependency 'httpclient', '~> 2.2'