Updated gemspec to use latest version of nokogiri (1.8.0) which works… (#52)

* Updated gemspec to use latest version of nokogiri (1.8.0) which works with Ruby 2.4. Modified example code slightly in README to match project folder name ('spec' instead of 'specs').

* Updated rubyzip version.

Author: Aralox

README.md

@@ -22,7 +22,7 @@ Creek can simply parse an Excel file by looping through the rows enumerator:
 
 ```ruby
 require 'creek'
-creek = Creek::Book.new 'specs/fixtures/sample.xlsx'
+creek = Creek::Book.new 'spec/fixtures/sample.xlsx'
 sheet = creek.sheets[0]
 
 sheet.rows.each do |row|

creek.gemspec

@@ -25,7 +25,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'rspec', '~> 3.6.0'
   spec.add_development_dependency 'pry'
 
-  spec.add_dependency 'nokogiri', '~> 1.7.0'
-  spec.add_dependency 'rubyzip', '>= 1.0.0'
+  spec.add_dependency 'nokogiri', '~> 1.8.0'
+  spec.add_dependency 'rubyzip', '>= 1.2.1'
   spec.add_dependency 'httparty', '~> 0.15.5'
 end