Updates the GitHub url in the readme and gemspec

The Travis icon in the readme actually breaks because of the old
url, updated the others for consistency.
Removes Coveralls setup from spechelper

Coveralls was removed in 95a13f4, but not al references were removed,
causing the spec to fail
Fix Dropbox links in image_onebox engine
Merge pull request #210 from bartj3/fix/specs

Removes Coveralls setup from spec helper
Merge pull request #209 from bartj3/master

Updates the GitHub url in the readme and gemspec
Merge pull request #214 from riking/patch-1

Fix Dropbox links in image_onebox engine
Make dropbox image matching more strict
Merge pull request #215 from riking/patch-1

Make dropbox image matching more strict
Remove hexpress dependency