Fix link to man page

Man page has moved to wkhtmltopdf.org
Merge pull request #53 from mdh/patch-1

Fix link to man page
Update wkhtml2pdf binary and bump version to 0.6.0.pre
bumping up nokogiri version to 1.6 so that we can use gimli with rails 4.2
upgrading a lot of other outdated gem dependancies
Upgraded all outdated gems, including the un-maintained wkhtmltopdf-binary gem. Test cases were failing due to change in syntax and need for explicit declaration of syntax style. Have made those changes too. Now all test cases are passing
removing outdated ruby versions to test against in travis CI - namely REE and Ruby 1.8.7.
Bump version number
Merge branch 'development'