Remove un-used Travis-CI-image in the README.
Remove un-used Travis-CI-image in the README.

assetgraph/assetgraph,papandreou/assetgraph,jscinoz/assetgraph,jscinoz/assetgraph,papandreou/assetgraph,slang800/assetgraph,papandreou/assetgraph,jscinoz/assetgraph,slang800/assetgraph