Pin Rake to ~> 10.1.0 to support Ruby 1.8.7

The latest Rake update requires Ruby >= 1.9. This update
fixes the failing 1.8.7 tests by pinning Rake to the last
supported version on ruby 1.8.7.
Add Travis build status to README
Merge pull request #77 from blkperl/fix_travis

Pin Rake to ~> 10.1 to support Ruby 1.8.7
Merge pull request #78 from blkperl/add_travis_image

Add Travis build status to README